Bump DavidAnson/markdownlint-cli2-action from 20 to 21

Bumps [DavidAnson/markdownlint-cli2-action](https://github.com/davidanson/markdownlint-cli2-action) from 20 to 21.
- [Release notes](https://github.com/davidanson/markdownlint-cli2-action/releases)
- [Commits](DavidAnson/markdownlint-cli2-action@v20...v21)

---
updated-dependencies:
- dependency-name: DavidAnson/markdownlint-cli2-action
  dependency-version: '21'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/main.yml

@@ -85,7 +85,7 @@ jobs:
     needs: [build]
     steps:
       - uses: actions/checkout@v5
-      - uses: DavidAnson/markdownlint-cli2-action@v20
+      - uses: DavidAnson/markdownlint-cli2-action@v21
         with:
           globs: |
             README.md