chore: Bump DavidAnson/markdownlint-cli2-action from 20 to 21 (#3396)

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/markdown-lint.yml

@@ -17,7 +17,7 @@ jobs:
           !data/planet/**/*.md
           !data/changelog/**/*.md
         separator: ","
-    - uses: DavidAnson/markdownlint-cli2-action@v20
+    - uses: DavidAnson/markdownlint-cli2-action@v21
       if: steps.changed-files.outputs.any_changed == 'true'
       with:
         globs: ${{ steps.changed-files.outputs.all_changed_files }}