build(deps): bump @slack/bolt from 3.12.2 to 3.15.0

Bumps [@slack/bolt](https://github.com/slackapi/bolt) from 3.12.2 to 3.15.0.
- [Release notes](https://github.com/slackapi/bolt/releases)
- [Commits](https://github.com/slackapi/bolt/compare/@slack/bolt@3.12.2...@slack/bolt@3.15.0)

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

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