chore(deps): update davidanson/markdownlint-cli2-action action to v20 (#26)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/lint-unit.yml

@@ -64,7 +64,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Markdown lint
-        uses: DavidAnson/markdownlint-cli2-action@v18
+        uses: DavidAnson/markdownlint-cli2-action@v20
         with:
           globs: "**/*.md"