Bump firebase-admin from 12.1.0 to 12.6.0 in /plugins/push

Bumps [firebase-admin](https://github.com/firebase/firebase-admin-node) from 12.1.0 to 12.6.0.
- [Release notes](https://github.com/firebase/firebase-admin-node/releases)
- [Changelog](https://github.com/firebase/firebase-admin-node/blob/master/CHANGELOG.md)
- [Commits](firebase/firebase-admin-node@v12.1.0...v12.6.0)

---
updated-dependencies:
- dependency-name: firebase-admin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]