chore(deps): bump slack-bolt from 1.20.1 to 1.21.2

Bumps [slack-bolt](https://github.com/slackapi/bolt-python) from 1.20.1 to 1.21.2.
- [Release notes](https://github.com/slackapi/bolt-python/releases)
- [Commits](slackapi/bolt-python@v1.20.1...v1.21.2)

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

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

requirements.txt

@@ -1,4 +1,4 @@
-slack-bolt==1.20.1
+slack-bolt==1.21.2
 requests==2.32.3
 Flask==3.0.3
 pytest==8.3.3