Bump google-auth-library from 9.15.0 to 9.15.1 in /scripts/gcb2slack (#3728)

Bumps [google-auth-library](https://github.com/googleapis/google-auth-library-nodejs) from 9.15.0 to 9.15.1.
- [Release notes](https://github.com/googleapis/google-auth-library-nodejs/releases)
- [Changelog](https://github.com/googleapis/google-auth-library-nodejs/blob/main/CHANGELOG.md)
- [Commits](googleapis/google-auth-library-nodejs@v9.15.0...v9.15.1)

---
updated-dependencies:
- dependency-name: google-auth-library
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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