[dependabot:actions] Bump citizensadvice/message-slack-action

Bumps [citizensadvice/message-slack-action](https://github.com/citizensadvice/message-slack-action) from 2 to 3.
- [Release notes](https://github.com/citizensadvice/message-slack-action/releases)
- [Commits](citizensadvice/message-slack-action@306ad8b...080d90b)

---
updated-dependencies:
- dependency-name: citizensadvice/message-slack-action
  dependency-version: '3'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/docker-image-scan.yml

@@ -75,7 +75,7 @@ jobs:
     if: failure()
     steps:
       - name: Notify Slack
-        uses: citizensadvice/message-slack-action@306ad8b4768287c24be25ebc9c0dd9d2af55a20a
+        uses: citizensadvice/message-slack-action@080d90bef86e16b0f1d9b07406fad8a9f541fc27
         with:
           slack_bot_token: ${{ secrets.SLACK_BOT_TOKEN }}
           github_token: ${{ secrets.GITHUB_TOKEN }}