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>

scripts/download_mmctl_release.sh

@@ -15,14 +15,7 @@ fi
 
 BIN_PATH=${2:-bin}
 
-# strip whitespace
-THIS_BRANCH=$(git rev-parse --abbrev-ref HEAD)
-if [[ "$THIS_BRANCH" =~ 'release-'[0-9] ]];
-then
-  RELEASE_TO_DOWNLOAD="$THIS_BRANCH"
-else
-  RELEASE_TO_DOWNLOAD=master
-fi
+RELEASE_TO_DOWNLOAD="release-6.0"
 
 echo "Downloading prepackaged binary: https://releases.mattermost.com/mmctl/$RELEASE_TO_DOWNLOAD";