pr-auditor: allow more base refs
Created by: bobheadxi
Adds "main", "master", "release" to the allowed base refs. This is purely an API call usage optimization, so we don't need to be so specific as to require usage to provide the default branch - we can just rely on a simple allowlist of commonly used default branches.
Enables https://github.com/sourcegraph/sourcegraph/pull/31687
Test plan
This PR should still get a status check