Help/About: Update the 5.3 About page.

* Remove the SVG from the upper right corner.
* Make sure admin notices that should be hidden stay hidden.
* Address a11y issues with contrast.
* Fix an issue with the active nav tab hover.
* Prevent names on the credits from breaking out of the box.
* Update two strings with a more accurate information.

Previously [46556], [46572], [46616].

Props ryelle, afercia, audrasjb, francina, mapk, desrosj, YordanSoares, melchoyce, azaozz, johnbillion, davidbaumwald, luminuu, bwmarkle, la-geek, SergeyBiryukov.
See #47708.
Built from https://develop.svn.wordpress.org/trunk@46705


git-svn-id: https://core.svn.wordpress.org/trunk@46505 1a063a9b-81f0-0310-95a4-ce76da25c4cd