GH Actions: Bump DavidAnson/markdownlint-cli2-action from 20 to 21

Bumps [DavidAnson/markdownlint-cli2-action](https://github.com/davidanson/markdownlint-cli2-action) from 20 to 21.
- [Release notes](https://github.com/davidanson/markdownlint-cli2-action/releases)
- [Commits](DavidAnson/markdownlint-cli2-action@v20...v21)

---
updated-dependencies:
- dependency-name: DavidAnson/markdownlint-cli2-action
  dependency-version: '21'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/markdown.yml

@@ -34,7 +34,7 @@ jobs:
 
       # @link https://github.com/marketplace/actions/markdownlint-cli2-action
       - name: Check markdown with CLI2
-        uses: DavidAnson/markdownlint-cli2-action@v20
+        uses: DavidAnson/markdownlint-cli2-action@v21
 
   remark:
     name: 'QA Markdown'