Bump markdownlint-cli2-action from v20 to v21

Author: petercrocker

.github/workflows/ci.yml

@@ -96,7 +96,7 @@ jobs:
       - name: "Check out repository code"
         uses: "actions/checkout@v6"
       - name: "Linting: markdownlint"
-        uses: DavidAnson/markdownlint-cli2-action@v20
+        uses: DavidAnson/markdownlint-cli2-action@v21
         with:
           config: .markdownlint.yaml
           globs: |