[fix] download mmctl release script

Going from `release-7.9` to `release-7.10,` has introduced a bug, in the download_mmctl_release.sh script, in which it is unable to o match version numbers with multiple digits
Specifically, `release-7.10` returned `release-7.1` downloading wrong mmctl binary.

We are refactoring the regular expression used, to match version numbers with multiple digits.

Ticket: https://mattermost.atlassian.net/browse/CLD-5682

server/scripts/download_mmctl_release.sh

@@ -15,13 +15,13 @@ 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=$(echo $THIS_BRANCH | grep -Eo 'release-([0-9](\.){0,1})\.([0-9](\.){0,1})')
+RELEASE_PATTERN='release-[0-9]+\.[0-9]+(\.[0-9]+)?'
+if [[ $THIS_BRANCH =~ $RELEASE_PATTERN ]]; then
+  RELEASE_TO_DOWNLOAD=$THIS_BRANCH
 else
-  RELEASE_TO_DOWNLOAD=master
+  RELEASE_TO_DOWNLOAD="master"
 fi
 
 echo "Downloading prepackaged binary: https://releases.mattermost.com/mmctl/$RELEASE_TO_DOWNLOAD";