Bump google-cloud-billing from 1.15.0 to 1.16.0

Bumps [google-cloud-billing](https://github.com/googleapis/google-cloud-python) from 1.15.0 to 1.16.0.
- [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](googleapis/google-cloud-python@google-cloud-billing-v1.15.0...google-cloud-billing-v1.16.0)

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

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

Author: dependabot[bot]

function-source/requirements.txt

@@ -1,3 +1,3 @@
 # Function dependencies, for example:
 # package>=version
-google-cloud-billing==1.15.0
+google-cloud-billing==1.16.0