[Infra] Require firestore status check only if firestore code changes

[ncooke3]

The Firestore status check should only be required on Firestore PRs.

Relevant docs: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/collaborating-on-repositories-with-code-quality-features/troubleshooting-required-status-checks#handling-skipped-but-required-checks

AC (to be tested post-merge):

• [ ] status check is skipped if changes != true
• [ ] jobs are skipped if changes != true

no-changelog

[ncooke3]

Tested it out in #13706 and it LGTM