replace our custom makrdowlint with DavidAnson/markdownlint-cli2-action

Contanerbuild markdown action is outdated and not working, instead of
fixing and duplicating work, use existing action.

Signed-off-by: Martin Basti <mbasti@redhat.com>

.github/workflows/linters.yaml

@@ -55,7 +55,7 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Run markdownlint
-        uses: containerbuildsystem/actions/markdownlint@master
+        uses: DavidAnson/markdownlint-cli2-action@v16
 
   mypy:
     name: mypy