Update custom url schemes error message to reflect what we actually support (#9061)

i18n/en.json

@@ -3784,7 +3784,7 @@
   },
   {
     "id": "model.config.is_valid.display.custom_url_schemes.app_error",
-    "translation": "The custom URL scheme {{.Scheme}} is invalid. Custom URL schemes must start with a letter and contain only letters, numbers, plus (+), period (.), and hyphen (-)."
+    "translation": "The custom URL scheme {{.Scheme}} is invalid. Custom URL schemes must start with a letter and contain only letters, numbers and hyphen (-)."
   },
   {
     "id": "model.config.is_valid.elastic_search.aggregate_posts_after_days.app_error",

model/config.go

@@ -2418,7 +2418,7 @@ func (mes *MessageExportSettings) isValid(fs FileSettings) *AppError {
 
 func (ds *DisplaySettings) isValid() *AppError {
 	if len(*ds.CustomUrlSchemes) != 0 {
-		validProtocolPattern := regexp.MustCompile(`(?i)^\s*[a-z][a-z0-9+.-]*\s*$`)
+		validProtocolPattern := regexp.MustCompile(`(?i)^\s*[a-z][a-z0-9-]*\s*$`)
 
 		for _, scheme := range *ds.CustomUrlSchemes {
 			if !validProtocolPattern.MatchString(scheme) {

model/config_test.go

@@ -466,7 +466,7 @@ func TestDisplaySettingsIsValidCustomUrlSchemes(t *testing.T) {
 		{
 			name:  "containing period",
 			value: []string{"iris.beep"},
-			valid: true,
+			valid: false, // should technically be true, but client doesn't support it
 		},
 		{
 			name:  "containing hyphen",
@@ -476,7 +476,7 @@ func TestDisplaySettingsIsValidCustomUrlSchemes(t *testing.T) {
 		{
 			name:  "containing plus",
 			value: []string{"coap+tcp", "coap+ws"},
-			valid: true,
+			valid: false, // should technically be true, but client doesn't support it
 		},
 		{
 			name:  "starting with number",