Help/About: String updates and accessibility improvements to the 5.5 About page.

Merges [48729-48730] to the 5.5 branch.
Props audrasjb, SergeyBiryukov.
See #50416.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48495 1a063a9b-81f0-0310-95a4-ce76da25c4cd