Bump dart-lang/setup-dart from 1.5.1 to 1.6.0

Bumps [dart-lang/setup-dart](https://github.com/dart-lang/setup-dart) from 1.5.1 to 1.6.0.
- [Release notes](https://github.com/dart-lang/setup-dart/releases)
- [Changelog](https://github.com/dart-lang/setup-dart/blob/main/CHANGELOG.md)
- [Commits](dart-lang/setup-dart@8a4b97e...b64355a)

---
updated-dependencies:
- dependency-name: dart-lang/setup-dart
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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