[main] Copy v20.0.0 release notes

[shlomi-noach]

This Pull Request copies the release notes found on release-20.0-rc to keep release notes up-to-date after the v20.0.0 release.

[vitess-bot]

Review Checklist

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

General

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Tests

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Documentation

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