chore(deps): bump google-api-python-client in /docs/mail-merge

Bumps [google-api-python-client](https://github.com/googleapis/google-api-python-client) from 2.31.0 to 2.151.0.
- [Release notes](https://github.com/googleapis/google-api-python-client/releases)
- [Commits](googleapis/google-api-python-client@v2.31.0...v2.151.0)

---
updated-dependencies:
- dependency-name: google-api-python-client
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/mail-merge/requirements.txt

@@ -1,3 +1,3 @@
 google-auth==2.3.3
-google-api-python-client==2.31.0
+google-api-python-client==2.151.0
 oauth2client==4.1.3