build(deps): bump slack-sdk from 3.18.3 to 3.19.1

Bumps [slack-sdk](https://github.com/slackapi/python-slack-sdk) from 3.18.3 to 3.19.1.
- [Release notes](https://github.com/slackapi/python-slack-sdk/releases)
- [Changelog](https://github.com/slackapi/python-slack-sdk/blob/main/docs-v2/changelog.html)
- [Commits](slackapi/python-slack-sdk@v3.18.3...v3.19.1)

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

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