Help/About: Update the 5.3 About page for RC2.

This includes numerous improvements:

- Removes the jazzer name.
- Ensures `WordPress` is translatable for consistency with other occurrences in Core.
- Changes all instances of `Block Editor` to `block editor` to match the spelling best practices.
- Updates the PHP 7.4 compatibility section with more suitable text.
- Introduction in `credits.php`, `freedoms.php`, and `privacy.php` have been updated to be consistent with the About page.
- Fixes the `aria-current` attribute in tab navigation.

Reviewed by SergeyBiryukov, desrosj.
Props SergeyBiryukov, audrasjb, desrosj, ryelle, afercia.
See #47708.
Built from https://develop.svn.wordpress.org/trunk@46572


git-svn-id: https://core.svn.wordpress.org/trunk@46369 1a063a9b-81f0-0310-95a4-ce76da25c4cd