Bump @grpc/grpc-js and @google-cloud/firestore

Bumps [@grpc/grpc-js](https://github.com/grpc/grpc-node) to 1.8.21 and updates ancestor dependency [@google-cloud/firestore](https://github.com/googleapis/nodejs-firestore). These dependencies need to be updated together.


Updates `@grpc/grpc-js` from 1.6.12 to 1.8.21
- [Release notes](https://github.com/grpc/grpc-node/releases)
- [Commits](https://github.com/grpc/grpc-node/compare/@grpc/[email protected]...@grpc/[email protected])

Updates `@google-cloud/firestore` from 4.15.1 to 6.7.0
- [Release notes](https://github.com/googleapis/nodejs-firestore/releases)
- [Changelog](https://github.com/googleapis/nodejs-firestore/blob/main/CHANGELOG.md)
- [Commits](googleapis/nodejs-firestore@v4.15.1...v6.7.0)

---
updated-dependencies:
- dependency-name: "@grpc/grpc-js"
  dependency-type: indirect
- dependency-name: "@google-cloud/firestore"
  dependency-type: direct:production
...

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