Accessibility: Update and standardize the admin screen-reader-text CSS class.

Given the new WordPress browsers support policy, the `screen-reader-text` css
class used in the admin can be updated to use modern CSS and correct syntax. See
https://github.com/wpaccessibility/a11ythemepatterns/blob/master/read-more-links/style.css

Worth noting the `clip` property is deprecated and kept for IE11 and Edge.

- uses `clip-path` for modern browsers
- keeps `clip` for old browsers and update its value to a correct syntax
- resets `clip-path` to `none` where the class is used to dynamically reveal elements
- removes an old rule that made `screen-reader-text` completely invisible in the help tabs `#screen-meta`
- standardizes the rule across CSS files

Fixes #40970.

Built from https://develop.svn.wordpress.org/trunk@41622


git-svn-id: http://core.svn.wordpress.org/trunk@41457 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/nav-menus.css

@@ -599,6 +599,7 @@ body.menu-max-depth-11 { min-width: 1280px !important; }
 
 .no-js.nav-menus-php .item-edit .screen-reader-text {
 	position: static;
+	clip-path: none;
 	width: auto;
 	height: auto;
 	margin: 0;