build(deps): update slack-bolt requirement

Updates the requirements on [slack-bolt](https://github.com/slackapi/bolt-python) to permit the latest version.
- [Release notes](https://github.com/slackapi/bolt-python/releases)
- [Commits](slackapi/bolt-python@v1.18.0...v1.28.0)

---
updated-dependencies:
- dependency-name: slack-bolt
  dependency-version: 1.28.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -1,2 +1,2 @@
-slack_bolt>=1.18.0,<2
+slack_bolt>=1.28.0,<2
 packaging<27