refactor!: retire legacy administration

BREAKING CHANGE: legacy management routes now fail closed and the legacy helper API is removed.

lang/en/local_webhooks.php

@@ -41,6 +41,7 @@ $string['eventwebserviceservicedeleted'] = 'Web service deleted';
 $string['eventwebserviceserviceupdated'] = 'Web service updated';
 $string['externalservice'] = 'External service';
 $string['file'] = 'File';
+$string['legacyuidisabled'] = 'Legacy webhook management is disabled during the 6.0 upgrade.';
 $string['name'] = 'Name';
 $string['pluginname'] = 'WebHooks';
 $string['privacy:metadata'] = 'WebHooks does not store any personal data.';