Bump slack-sdk from 3.27.1 to 3.27.2 in /backend

Bumps [slack-sdk](https://github.com/slackapi/python-slack-sdk) from 3.27.1 to 3.27.2.
- [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@v3.27.1...v3.27.2)

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

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

backend/requirements.txt

@@ -27,7 +27,7 @@ PyYAML==6.0.1
 requests==2.31.0
 requests-mock==1.11.0
 slack_bolt==1.18.1
-slack_sdk==3.27.1
+slack_sdk==3.27.2
 sqlalchemy==1.4.52
 waitress==3.0.0
 werkzeug==3.0.3