ci: bump markdownlint-cli2-action from 5.1.0 to 6.0.0 (#632)

Links:
- https://github.com/DavidAnson/markdownlint-cli2-action/releases/tag/v6.0.0
- DavidAnson/markdownlint-cli2-action@744f913...d199b6e

.github/workflows/markdownlint.yml

@@ -11,6 +11,6 @@ jobs:
         uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b
 
       - name: Run markdownlint
-        uses: DavidAnson/markdownlint-cli2-action@744f913a124058ee903768d3adb92a4847e5d132
+        uses: DavidAnson/markdownlint-cli2-action@d199b6e1b89360c71e0c21eac02f7965faf07ba6
         with:
           globs: "**/*.md"