Lock CI for release-6.0 (#17756)

* Lock CI for release-6.0

* Update download_mmctl_release.sh

Co-authored-by: Mmbot <mmbot@mattermost>
Co-authored-by: Elisabeth Kulzer <elikul@elikul.de>

.gitlab-ci.yml

@@ -7,7 +7,7 @@ stages:
 
 include:
   - project: mattermost/ci/mattermost-server
-    ref: master
+    ref: release-6.0
     file: private.yml
 
 variables: