[main] Copy v20.0.3 release notes

[vitess-bot]

This Pull Request copies the release notes found on release-20.0 to keep release notes up-to-date after the v20.0.3 release.

[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.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

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• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

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If a workflow is added or modified:

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Backward compatibility

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• vtctl command output order should be stable and awk-able.