Bump ajv, @google-cloud/firestore and request

Bumps [ajv](https://github.com/ajv-validator/ajv), [@google-cloud/firestore](https://github.com/googleapis/nodejs-firestore) and [request](https://github.com/request/request). These dependencies needed to be updated together.

Updates `ajv` from 4.11.8 to 6.12.6
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](ajv-validator/ajv@4.11.8...v6.12.6)

Updates `@google-cloud/firestore` from 0.13.0 to 0.13.1
- [Release notes](https://github.com/googleapis/nodejs-firestore/releases)
- [Changelog](https://github.com/googleapis/nodejs-firestore/blob/main/CHANGELOG.md)
- [Commits](googleapis/nodejs-firestore@v0.13.0...v0.13.1)

Updates `request` from 2.85.0 to 2.88.2
- [Changelog](https://github.com/request/request/blob/master/CHANGELOG.md)
- [Commits](https://github.com/request/request/commits)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
- dependency-name: "@google-cloud/firestore"
  dependency-type: indirect
- dependency-name: request
  dependency-type: indirect
...

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