Accessibility: Administration: Address backward compatibility concerns for the...

Accessibility: Administration: Address backward compatibility concerns for the sortable postboxes areas after [48340].

Merges [48717] to the 5.5 branch.
Fixes #49288 for 5.5.

Built from https://develop.svn.wordpress.org/branches/5.5@48718


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48480 1a063a9b-81f0-0310-95a4-ce76da25c4cd