chore(deps): bump DavidAnson/markdownlint-cli2-action from 17 to 21

Bumps [DavidAnson/markdownlint-cli2-action](https://github.com/davidanson/markdownlint-cli2-action) from 17 to 21.
- [Release notes](https://github.com/davidanson/markdownlint-cli2-action/releases)
- [Commits](DavidAnson/markdownlint-cli2-action@v17...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] <support@github.com>

Author: dependabot[bot]

.github/workflows/lint.yml

@@ -11,13 +11,13 @@ jobs:
     steps:
       - uses: actions/checkout@v5
       - name: Markdownlint
-        uses: DavidAnson/markdownlint-cli2-action@v20
+        uses: DavidAnson/markdownlint-cli2-action@v21
         with:
           globs: "**/*.md !volumes/** !book/** !site/** !.github/ISSUE_TEMPLATE/** !.github/PULL_REQUEST_TEMPLATE.md"
 
       # Markdown lint
       - name: markdownlint
-        uses: DavidAnson/markdownlint-cli2-action@v17
+        uses: DavidAnson/markdownlint-cli2-action@v21
         with:
           globs: "**/*.md !volumes/** !book/** !site/** !.github/ISSUE_TEMPLATE/** !.github/PULL_REQUEST_TEMPLATE.md"