chore: bump DavidAnson/markdownlint-cli2-action from 20 to 21 (#1790)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/ci-markdown-checks.yml

@@ -19,7 +19,7 @@ jobs:
 
       # equivalent cli: markdownlint-cli2  "**/*.md" "#**/CHANGELOG.md"  --config .markdownlint.json
       - name: "Markdown Lint Check"
-        uses: DavidAnson/markdownlint-cli2-action@v20
+        uses: DavidAnson/markdownlint-cli2-action@v21
         continue-on-error: true
         with:
           fix: false