Update lerna monorepo to ~3.21.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
@lerna/project	dependencies	minor	~3.18.0 -> ~3.21.0
lerna	devDependencies	minor	3.20.2 -> 3.21.0

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Release Notes

lerna/lerna

### [`v3.21.0`](https://togithub.com/lerna/lerna/blob/master/CHANGELOG.md#​3210-httpsgithubcomlernalernacomparev3202v3210-2020-05-13) [Compare Source](https://togithub.com/lerna/lerna/compare/v3.18.0...v3.21.0) ##### Bug Fixes - **publish:** Canary releases without previous tags should not explode ([c9eb590](https://togithub.com/lerna/lerna/commit/c9eb590bf867889a188598322bb4552db7a34dfc)) ##### Features - **project:** Add `getPackagesSync()` export ([068bdd7](https://togithub.com/lerna/lerna/commit/068bdd75683ca9687a420523e4ba007bd8b3b220)) - **version:** Ignore private packages completely with `--no-private` ([a9b9f97](https://togithub.com/lerna/lerna/commit/a9b9f97457e4e4b0cac7f4ce562458d921a1f9be)) #### [3.20.2](https://togithub.com/lerna/lerna/compare/v3.20.1...v3.20.2) (2020-01-02) ##### Bug Fixes - **version:** Loosen `--atomic` fallback to catch incompatible CLI versions ([6f0e2bb](https://togithub.com/lerna/lerna/commit/6f0e2bb1b033b0579910cedcf0be84f1474c1580)), closes [#​2400](https://togithub.com/lerna/lerna/issues/2400) #### [3.20.1](https://togithub.com/lerna/lerna/compare/v3.20.0...v3.20.1) (2019-12-29) ##### Bug Fixes - **version:** Support git clients that do not support `git push --atomic` ([2b9b210](https://togithub.com/lerna/lerna/commit/2b9b210c0b6ac69853ffb01f0dbac9109ab419c5))

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