Bump google-cloud-kms from 2.19.1 to 2.20.0 in /docker/google-kms

Bumps [google-cloud-kms](https://github.com/googleapis/google-cloud-python) from 2.19.1 to 2.20.0.
- [Release notes](https://github.com/googleapis/google-cloud-python/releases)
- [Changelog](https://github.com/googleapis/google-cloud-python/blob/main/packages/google-cloud-documentai/CHANGELOG.md)
- [Commits](https://github.com/googleapis/google-cloud-python/commits/google-cloud-kms-v2.20.0)

---
updated-dependencies:
- dependency-name: google-cloud-kms
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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