Upodate markdownlint action

Author: aversey

.github/workflows/mkdocs-test.yml

@@ -21,7 +21,7 @@ jobs:
           path: hopsworks-api
 
       - name: Markdownlint
-        uses: DavidAnson/markdownlint-cli2-action@v9
+        uses: DavidAnson/markdownlint-cli2-action@v21
         with:
           globs: '**/*.md'