Help/About: Final adjustments to the 5.5 About page.

- Final CDN asset URLs.
- Some minor color adjustments.

Props ryelle, marybaum, davidbaumwald, audrasjb, estelaris, afercia, cbringmann, andreamiddleton, desrosj, SergeyBiryukov, ocean90, nrqsnchz, spacedmonkey, whyisjake, man4toman.
Merges [48783] to the 5.5 branch.
Fixes #50416.
Built from https://develop.svn.wordpress.org/branches/5.5@48784


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48546 1a063a9b-81f0-0310-95a4-ce76da25c4cd