chore(deps): bump slack-bolt from 1.20.1 to 1.21.1

Bumps [slack-bolt](https://github.com/slackapi/bolt-python) from 1.20.1 to 1.21.1.
- [Release notes](https://github.com/slackapi/bolt-python/releases)
- [Commits](slackapi/bolt-python@v1.20.1...v1.21.1)

---
updated-dependencies:
- dependency-name: slack-bolt
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

requirements-base.txt

@@ -396,7 +396,7 @@ six==1.16.0
     #   python-dateutil
     #   sqlalchemy-filters
     #   validators
-slack-bolt==1.20.1
+slack-bolt==1.21.1
     # via -r requirements-base.in
 slack-sdk==3.33.1
     # via