api4/license: Return 400 instead of 501 if format is not passed (#19856)

```release-note
NONE
```


Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/license.go

@@ -31,7 +31,7 @@ func getClientLicense(c *Context, w http.ResponseWriter, r *http.Request) {
 	format := r.URL.Query().Get("format")
 
 	if format == "" {
-		c.Err = model.NewAppError("getClientLicense", "api.license.client.old_format.app_error", nil, "", http.StatusNotImplemented)
+		c.Err = model.NewAppError("getClientLicense", "api.license.client.old_format.app_error", nil, "", http.StatusBadRequest)
 		return
 	}
 

api4/license_test.go

@@ -38,8 +38,8 @@ func TestGetOldClientLicense(t *testing.T) {
 
 	resp, err := client.DoAPIGet("/license/client", "")
 	require.Error(t, err, "get /license/client did not return an error")
-	require.Equal(t, http.StatusNotImplemented, resp.StatusCode,
+	require.Equal(t, http.StatusBadRequest, resp.StatusCode,
-		"expected 501 Not Implemented")
+		"expected 400 bad request")
 
 	resp, err = client.DoAPIGet("/license/client?format=junk", "")
 	require.Error(t, err, "get /license/client?format=junk did not return an error")