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

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

No files, tests, or workflow entry points are named in the issue, so start by locating the release workflow and the checks that create or merge release PRs. Done means out-of-order release workflows cannot run and a release PR cannot merge while earlier release PRs remain open; add tests or validation for both cases.

Written by the indexing model from the issue text.