Add ci_jwt_signing_key to list of tokens to delete

doc/raketasks/backup_restore.md

@@ -1285,6 +1285,7 @@ You may need to reconfigure or restart GitLab for the changes to take effect.
    UPDATE namespaces SET runners_token = null, runners_token_encrypted = null;
    -- Clear instance tokens
    UPDATE application_settings SET runners_registration_token_encrypted = null;
+   UPDATE application_settings SET ci_jwt_signing_key = null;
    -- Clear runner tokens
    UPDATE ci_runners SET token = null, token_encrypted = null;
    ```