[batch-release] A GA workflow to sync release branch back to master on merges to release branch

[chunhtai]

main doc: flutter.dev/go/batch-release-one-pager

This is the last step of the release process, we need a workflow that listens to commit merged in release branch and create a pr to pull release back to main

step1: #176425

step2: #176944

step3: this issue