Improve keyboard control of Edit Selection mode in the media manager.

See #29326
Props adamsilverstein

Built from https://develop.svn.wordpress.org/trunk@30374


git-svn-id: http://core.svn.wordpress.org/trunk@30371 1a063a9b-81f0-0310-95a4-ce76da25c4cd
WPDB: When a `db.php` drop-in is being used, and it doesn't explicitly define itself as connecting to MySQL, skip the character set checks. This ensures that existing drop-ins won't accidentally run checks that they don't support.

See #21212.

Built from https://develop.svn.wordpress.org/trunk@30375


git-svn-id: http://core.svn.wordpress.org/trunk@30372 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve Thickbox's close button so it can be announced by screen readers.

Fixes #29455
Props SergeyBiryukov

Built from https://develop.svn.wordpress.org/trunk@30376


git-svn-id: http://core.svn.wordpress.org/trunk@30373 1a063a9b-81f0-0310-95a4-ce76da25c4cd