Bump markdownlint-cli2 from 0.20.0 to 0.21.0

Bumps [markdownlint-cli2](https://github.com/DavidAnson/markdownlint-cli2) from 0.20.0 to 0.21.0.
- [Changelog](https://github.com/DavidAnson/markdownlint-cli2/blob/main/CHANGELOG.md)
- [Commits](https://github.com/DavidAnson/markdownlint-cli2/compare/v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: markdownlint-cli2
  dependency-version: 0.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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