Bump actions/checkout from 3.0.2 to 3.2.0 (dart-lang/markdown#501)

Bumps [actions/checkout](https://github.com/actions/checkout) from 3.0.2 to 3.2.0.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v3.0.2...755da8c3cf115ac066823e79a1e1788f8940201b)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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