chore(deps): update davidanson/markdownlint-cli2-action action to v21 (#46)

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

Author: renovate[bot]

.github/workflows/markdownlint.yml

@@ -18,6 +18,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
-      - uses: DavidAnson/markdownlint-cli2-action@v20
+      - uses: DavidAnson/markdownlint-cli2-action@v21
         with:
           globs: '**/*.md'