.github: Bump ruby/action-slack from 3.0.0 to 3.2.1

Bumps [ruby/action-slack](https://github.com/ruby/action-slack) from 3.0.0 to 3.2.1.
- [Release notes](https://github.com/ruby/action-slack/releases)
- [Commits](https://github.com/ruby/action-slack/compare/v3.0.0...v3.2.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/test.yml

@@ -450,7 +450,7 @@ jobs:
       # By having the job always run, but skipping its only step as needed, we
       # let the CI output collapse nicely in PRs.
       if: failure() && github.event_name == 'push'
-      uses: ruby/action-slack@v3.0.0
+      uses: ruby/action-slack@v3.2.1
       with:
         payload: |
           {