Bundled Themes: Make checkbox in comment form larger Twenty Twenty.

This makes the checkbox in the comment form larger and more consistent with other checkboxes.

Props kharisblank, audrasjb.
Fixes #48652.
Built from https://develop.svn.wordpress.org/trunk@46844


git-svn-id: https://core.svn.wordpress.org/trunk@46644 1a063a9b-81f0-0310-95a4-ce76da25c4cd