pip(deps): bump slack-sdk in /cmd/cloud-run/slack-message-sender (#12778)

Bumps [slack-sdk](https://github.com/slackapi/python-slack-sdk) from 3.21.3 to 3.34.0.
- [Release notes](https://github.com/slackapi/python-slack-sdk/releases)
- [Commits](slackapi/python-slack-sdk@v3.21.3...v3.34.0)

---
updated-dependencies:
- dependency-name: slack-sdk
  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>

Author: dependabot[bot]

cmd/cloud-run/slack-message-sender/requirements.txt

@@ -1,5 +1,5 @@
 slack_bolt==1.18.0
-slack_sdk==3.21.3
+slack_sdk==3.34.0
 flask>=2.3.2
 cloudevents==1.9.0
 gunicorn==22.0.0