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

[vitess-bot]

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

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.49%. Comparing base (3d4f41d) to head (841e860).
Report is 1 commits behind head on release-21.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17325      +/-   ##
================================================
- Coverage         67.51%   67.49%   -0.02%     
================================================
  Files              1570     1570              
  Lines            251411   251411              
================================================
- Hits             169744   169701      -43     
- Misses            81667    81710      +43     

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[frouioui]

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