build(deps): Bump google-cloud-storage from 1.20.0 to 1.21.0 in /bot

Bumps [google-cloud-storage](https://github.com/GoogleCloudPlatform/google-cloud-python) from 1.20.0 to 1.21.0.
- [Release notes](https://github.com/GoogleCloudPlatform/google-cloud-python/releases)
- [Changelog](https://github.com/googleapis/google-cloud-python/blob/master/CHANGELOG.md)
- [Commits](googleapis/google-cloud-python@storage-1.20.0...storage-1.21.0)

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

Author: dependabot-preview[bot]

bot/requirements.txt

@@ -1,5 +1,5 @@
 -e ../tools #egg=code-coverage-tools
-google-cloud-storage==1.20.0
+google-cloud-storage==1.21.0
 libmozdata==0.1.59
 pytoml==0.1.21
 pytz==2019.3