Prevent release workflow from being run if the PR for a previous release has not been merged yet

[mcmire]

If multiple release PRs are created and one of them is merged out of order, the resulting workflow may complete successfully, but then an attempt to run the workflow for the previous release PRs may fail.

We should prevent this from happening. We can do this by:

• Preventing a release workflow from running if the root package version is being bumped by more than 1
• Preventing a release PR from being merged if any PRs for previous releases are open