[release-21.0] Bump to `v21.0.2-SNAPSHOT` after the `v21.0.1` release

[frouioui]

Includes the changes required to go back into dev mode (v21.0.2-SNAPSHOT) after the release of v21.0.1.

[vitess-bot[bot]]

Review Checklist

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General

• [ ] Ensure that the Pull Request has a descriptive title.
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Tests

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Documentation

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