Bundled Themes: Bump version number and update changelog in Twenty Twenty for WordPress 5.3.1.

This bumps the Twenty Twenty version number to `1.1` and update the `readme.txt` changelog.

Props audrasjb, sinatrateam, SergeyBiryukov
Fixes #48944.
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git-svn-id: https://core.svn.wordpress.org/trunk@46702 1a063a9b-81f0-0310-95a4-ce76da25c4cd