ci: Force kerberos test to GA again

Replaying https://github.com/nextcloud/server/pull/43210
Which was accidentally reverted by
https://github.com/nextcloud/server/pull/43515

Signed-off-by: Joas Schilling <coding@schilljs.com>

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

               - '**.php'
 
   files-external-smb-kerberos:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     needs: changes
 
     if: ${{ github.repository_owner != 'nextcloud-gmbh' && needs.changes.outputs.src != 'false' }}