Bump slack-bolt from 1.20.1 to 1.21.2 (#198)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>