ci: make stable branch work

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

Author: susnux

.github/workflows/files-external-smb-kerberos.yml

@@ -56,6 +56,7 @@ jobs:
         with:
           persist-credentials: false
           repository: nextcloud/user_saml
+          ref: stable-6
           path: apps/user_saml
 
       - name: Install user_saml