chore(deps): bump slackapi/slack-github-action from 1.19.0 to 1.21.0

Bumps [slackapi/slack-github-action](https://github.com/slackapi/slack-github-action) from 1.19.0 to 1.21.0.
- [Release notes](https://github.com/slackapi/slack-github-action/releases)
- [Commits](https://github.com/slackapi/slack-github-action/compare/v1.19.0...v1.21.0)

---
updated-dependencies:
- dependency-name: slackapi/slack-github-action
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

.github/workflows/upload-assets-to-cdn.yml

@@ -50,7 +50,7 @@ jobs:
           aws s3 sync --cache-control 'public,max-age=86400' --exclude "*" --include "*.pdf" --content-disposition attachment   assets/legal s3://fxa-content-cdn-prod-distbucket-gqg70i8xqycy/legal
 
       - name: "Post to fxa-team Slack channel"
-        uses: slackapi/slack-github-action@v1.19.0
+        uses: slackapi/slack-github-action@v1.21.0
         with:
           channel-id: 'CLV3KMZ8B'
           slack-message: "New assets have been uploaded to CDN: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}"