Update DavidAnson/markdownlint-cli2-action action to v21

Author: renovate[bot]

.github/workflows/markdown-lint.yml

@@ -16,4 +16,4 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v5
-      - uses: DavidAnson/markdownlint-cli2-action@v20.0.0
+      - uses: DavidAnson/markdownlint-cli2-action@v21.0.0