Bump slack-bolt from 1.19.1 to 1.20.0

Bumps [slack-bolt](https://github.com/slackapi/bolt-python) from 1.19.1 to 1.20.0.
- [Release notes](https://github.com/slackapi/bolt-python/releases)
- [Commits](slackapi/bolt-python@v1.19.1...v1.20.0)

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

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