go-librarian strategy erroneously updating PR with both v1/v2

It was noticed recently that for our individual release configs where there is both a V1 and a V2 version of the library, release-please is erroneously going back and forth and switching the pull request between the two. On the V1 branch, there must be a bug in the branch selection logic. https://github.com/googleapis/google-cloud-go/pull/20127

Related: #2844

Start by tracing the go-librarian strategy's branch-selection path and compare its behavior for release configurations containing both V1 and V2. Reproduce the PR switching described in #20127; done means the release PR remains associated with the correct branch and version instead of alternating between them.

Written by the indexing model from the issue text.