Bump markdownlint_cli2_action from v20 to v21

Author: cakebaker

.github/workflows/CICD.yml

@@ -149,7 +149,7 @@ jobs:
       shell: bash
       run: |
         RUSTDOCFLAGS="-Dwarnings" cargo doc  ${{ steps.vars.outputs.CARGO_FEATURES_OPTION }} --no-deps --workspace --document-private-items
-    - uses: DavidAnson/markdownlint-cli2-action@v20
+    - uses: DavidAnson/markdownlint-cli2-action@v21
       with:
         fix: "true"
         globs: |