[google-auth-library-java] Investigate need for branch configs in release-please-config.yaml.

Some versions in `.github/release-please.yaml` seem to be outdated.

> not something we need to fix in here, but I assume that quite a few of these versions are no longer needed. Do you know which versions we can just remove from here?

_Originally posted by @lqiu96 in https://github.com/googleapis/google-auth-library-java/pull/1852#discussion_r2603325082_

Start by reviewing `.github/release-please.yaml` and the `release-please-config.yaml` branch configuration referenced in the title. Identify which listed versions are outdated or no longer needed, then verify the conclusion against the google-auth-library-java release setup. Done means clearly documenting the versions to remove and updating the relevant configuration if removal is confirmed.

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