Accessibility: Audit usage of abbreviations.

Title attributes on abbreviations are only available to a minority of users. The
best option is to avoid unnecessary abbreviations when possible. In the other cases,
use an `<abbr>` element (which provides a hint to user agents on how to announce
and display the abbreviation) and provide an expansion in plain text on first use.

- `readme.html`: improves abbreviations and removes unnecessary `title` attributes
- options-general: removes unnecessary abbreviations and improves the remaining ones
- customizer schedule changeset date: removes unnecessary abbreviations and improves the remaining ones
- posts table date: uses a `span` element instead of an `abbr` element because this is not an abbreviation

Fixes #46980.

Built from https://develop.svn.wordpress.org/trunk@45930


git-svn-id: http://core.svn.wordpress.org/trunk@45741 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/css/admin-bar-rtl.css

@@ -1065,7 +1065,6 @@ html:lang(he-il) .rtl #wpadminbar * {
 		to 100% viewport width at responsive sizes. */
 	#wpadminbar .ab-top-menu > .menupop > .ab-sub-wrapper {
 		min-width: -webkit-fit-content;
-		min-width: -moz-fit-content;
 		min-width: fit-content;
 	}
 

wp-includes/css/admin-bar.css

@@ -1065,7 +1065,6 @@ html:lang(he-il) .rtl #wpadminbar * {
 		to 100% viewport width at responsive sizes. */
 	#wpadminbar .ab-top-menu > .menupop > .ab-sub-wrapper {
 		min-width: -webkit-fit-content;
-		min-width: -moz-fit-content;
 		min-width: fit-content;
 	}