Prevent synced CPA values to be updated from the API (#30687)

* Prevents the API from updating synced CPA values

The patch functions for CPA values now accept a parameter that checks
if they should allow for synced values to be updated, and prevent
those updates if necessary

* Fix linter

* Fix parameter after merge

---------

Co-authored-by: Miguel de la Cruz <miguel@ctrlz.es>
Co-authored-by: Scott Bishel <scott.bishel@mattermost.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/custom_profile_attributes.go

@@ -226,7 +226,7 @@ func patchCPAValues(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	results := make(map[string]json.RawMessage, len(updates))
 	for fieldID, rawValue := range updates {
-		patchedValue, appErr := c.App.PatchCPAValue(userID, fieldID, rawValue)
+		patchedValue, appErr := c.App.PatchCPAValue(userID, fieldID, rawValue, false)
 		if appErr != nil {
 			c.Err = appErr
 			return