chore(deps): update davidanson/markdownlint-cli2-action action to v21 (#5156)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/lint.yml

@@ -15,7 +15,7 @@ jobs:
         uses: actions/checkout@v5
 
       - name: Run markdownlint
-        uses: DavidAnson/markdownlint-cli2-action@v20
+        uses: DavidAnson/markdownlint-cli2-action@v21
         with:
           globs: |
             *.md