Bump DavidAnson/markdownlint-cli2-action from 20 to 21 (#1)

Dependency update

Author: dependabot[bot]

.github/workflows/lint.yml

@@ -112,7 +112,7 @@ jobs:
         uses: actions/checkout@v5
 
       - name: Markdown Lint
-        uses: DavidAnson/markdownlint-cli2-action@v20
+        uses: DavidAnson/markdownlint-cli2-action@v21
         with:
           globs: |
             **/*.md