match-lms-release: Check if maintenance branch already exists

[svanherk]

If the match-lms-release action fails when pushing the update for a new release version, it'll get into a state where the maintenance branch was created but the version is still on the previous (not current) release. And then it will be stuck in this state because creating the maintenance branch will fail: https://github.com/Brightspace/insights-adoption-dashboard/actions/runs/2833433713. I think we can avoid this by just checking if the branch was already created.

You could also delete the maintenance branch to get around this, but in the example above they'd already pushed a change to it so it couldn't just be recreated.