Move the milestone-proposed logic to tag script

The release pipeline does not have any branch information, because tag
events are branchless. So, in the script, check to see if the tag is in
milestone-proposed, and if so, submit the merge commit.

Change-Id: Ia46bf6ad7826571d39884cf7cd8165c5cea678e6
Merge "Move the milestone-proposed logic to tag script"