MM-43202: Change some 501 response codes to fix SLO violations (#20767)

There were lots of cases where we were using the code 501
in an incorrect manner, and since 5xx codes are considered
as SLO violations, these were flagged by our monitoring incorrectly.

These were the guidelines to change the codes:
1. Any case of missing license or config not correctly enabled
is set as Forbidden. The logic is that the user is correctly
authenticated with a session, but does not have enough privileges
to have the higher license, or hasn't changed the config correctly.
2. Any case of incorrectly formed request is obviously a 400.

https://mattermost.atlassian.net/browse/MM-43202

```release-note
NONE
```

api4/cloud.go

@@ -73,7 +73,7 @@ func handleNotifyAdminToUpgrade(c *Context, w http.ResponseWriter, r *http.Reque
 
 func getSubscription(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.getSubscription", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.getSubscription", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -296,7 +296,7 @@ func validateWorkspaceBusinessEmail(c *Context, w http.ResponseWriter, r *http.R
 
 func getCloudProducts(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.getCloudProducts", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.getCloudProducts", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -338,7 +338,7 @@ func getCloudProducts(c *Context, w http.ResponseWriter, r *http.Request) {
 
 func getCloudLimits(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.getCloudLimits", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.getCloudLimits", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -359,7 +359,7 @@ func getCloudLimits(c *Context, w http.ResponseWriter, r *http.Request) {
 
 func getCloudCustomer(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.getCloudCustomer", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.getCloudCustomer", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -385,7 +385,7 @@ func getCloudCustomer(c *Context, w http.ResponseWriter, r *http.Request) {
 
 func updateCloudCustomer(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.updateCloudCustomer", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.updateCloudCustomer", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -423,7 +423,7 @@ func updateCloudCustomer(c *Context, w http.ResponseWriter, r *http.Request) {
 
 func updateCloudCustomerAddress(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.updateCloudCustomerAddress", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.updateCloudCustomerAddress", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -461,7 +461,7 @@ func updateCloudCustomerAddress(c *Context, w http.ResponseWriter, r *http.Reque
 
 func createCustomerPayment(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.createCustomerPayment", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.createCustomerPayment", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -492,7 +492,7 @@ func createCustomerPayment(c *Context, w http.ResponseWriter, r *http.Request) {
 
 func confirmCustomerPayment(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.confirmCustomerPayment", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.confirmCustomerPayment", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -529,7 +529,7 @@ func confirmCustomerPayment(c *Context, w http.ResponseWriter, r *http.Request)
 
 func getInvoicesForSubscription(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.getInvoicesForSubscription", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.getInvoicesForSubscription", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -555,7 +555,7 @@ func getInvoicesForSubscription(c *Context, w http.ResponseWriter, r *http.Reque
 
 func getSubscriptionInvoicePDF(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.getSubscriptionInvoicePDF", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.getSubscriptionInvoicePDF", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}
 
@@ -590,7 +590,7 @@ func getSubscriptionInvoicePDF(c *Context, w http.ResponseWriter, r *http.Reques
 
 func handleCWSWebhook(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.App.Channels().License() == nil || !*c.App.Channels().License().Features.Cloud {
-		c.Err = model.NewAppError("Api4.handleCWSWebhook", "api.cloud.license_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("Api4.handleCWSWebhook", "api.cloud.license_error", nil, "", http.StatusForbidden)
 		return
 	}