This is an attempt to automatically sync our gem versions after
release-please detects the need to bump the version. It will open up a
PR with that new version bump to judoscale-ruby and changelog added, and
these extra steps should follow-up that with syncing the new version
across the other libraries.

I have done some manual testing on this using a separate branch, by
commenting out the release-please and "if" statements, and only
triggering these actual steps that sync versions and push a new commit.
By bumping the judoscale-ruby version myself in a commit, the automated
steps here would generate a new follow-up commit automatically syncing
the other gems versions, so I'm hopeful this will work as expected.
(famous last words)

The git user name/email and commands in general are based on examples:
https://github.com/actions/checkout?tab=readme-ov-file#push-a-commit-using-the-built-in-token
https://github.com/google-github-actions/release-please-action/tree/v3.7.13?tab=readme-ov-file#creating-majorminor-tags

We need to checkout the release PR to sync versions, otherwise it might
use `main` which is where this workflow is running.

It seems the "outputs" object has access to a PR object if one was
created (which it'd have been in this case):
https://github.com/google-github-actions/release-please-action?tab=readme-ov-file#outputs

and that PR object has access to both head & base branch names:
https://github.com/googleapis/release-please/blob/cb106cd0b5ae5c3e2a3b015487dc615eba72b842/src/pull-request.ts#L15-L17

so I believe we can checkout the right branch for that PR and run the
sync/commit there, with this change.

It's hard to test all of this without actually running a release though,
so we might have to wait and see! (and hope!)