Bump lerna from 3.20.2 to 3.21.0

Bumps [lerna](https://github.com/lerna/lerna/tree/HEAD/core/lerna) from 3.20.2 to 3.21.0.
- [Release notes](https://github.com/lerna/lerna/releases)
- [Changelog](https://github.com/lerna/lerna/blob/master/core/lerna/CHANGELOG.md)
- [Commits](https://github.com/lerna/lerna/commits/v3.21.0/core/lerna)

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