Bump davidanson/markdownlint-cli2-action from 20.0.0 to 21.0.0 (#135)

Bumps [davidanson/markdownlint-cli2-action](https://github.com/davidanson/markdownlint-cli2-action) from 20.0.0 to 21.0.0.
- [Release notes](https://github.com/davidanson/markdownlint-cli2-action/releases)
- [Commits](DavidAnson/markdownlint-cli2-action@v20.0.0...v21.0.0)

---
updated-dependencies:
- dependency-name: davidanson/markdownlint-cli2-action
  dependency-version: 21.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -12,6 +12,6 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v5.0.1
       - name: Check formatting
-        uses: davidanson/markdownlint-cli2-action@v20.0.0
+        uses: davidanson/markdownlint-cli2-action@v21.0.0
         with:
           globs: '**/*.md'