Apiary updates have become difficult & painful

1. Recently, apiary updates have started failing, see our [action log](https://github.com/googleapis/google-api-nodejs-client/actions/workflows/update-apis.yaml). Some of these were due to Node 18 see (fixes #3685, #3686), but others seem to mysteriously fail with "Unprocessable Entity".
2. The action often will result in a PR that looks fine, but when you go to merge it says: "This branch cannot be rebased". However it will not suggest the normal conflict-resolution suggestions, and so far the only way I have solved this is by manually-merging.
3. Release PRs must be merged via a Squash strategy for release-please, but generator PRs need to be merged via Rebase. Github just recommends the latest way of merging, and there are no restrictions/rules to this back-and-forth strategy changing.

Start with the update-apis.yaml workflow and its action log to identify the Unprocessable Entity failures and Node 18-related behavior. Then inspect the generated PR merge failures and the differing squash versus rebase requirements for release-please and generator PRs. Done means the Apiary update workflow completes reliably and the required merge strategy is clear and workable.

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