[MM-54029] IDP descriptor can any string (#24250)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/i18n/en.json

@@ -8860,7 +8860,7 @@
   },
   {
     "id": "model.config.is_valid.saml_idp_descriptor_url.app_error",
-    "translation": "Identity Provider Issuer URL must be a valid URL and start with http:// or https://."
+    "translation": "Identity Provider Issuer URL cannot be an empty string."
   },
   {
     "id": "model.config.is_valid.saml_idp_url.app_error",

server/public/model/config.go

@@ -3794,7 +3794,7 @@ func (s *SamlSettings) isValid() *AppError {
 			return NewAppError("Config.IsValid", "model.config.is_valid.saml_idp_url.app_error", nil, "", http.StatusBadRequest)
 		}
 
-		if *s.IdpDescriptorURL == "" || !IsValidHTTPURL(*s.IdpDescriptorURL) {
+		if *s.IdpDescriptorURL == "" {
 			return NewAppError("Config.IsValid", "model.config.is_valid.saml_idp_descriptor_url.app_error", nil, "", http.StatusBadRequest)
 		}