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

@@ -691,12 +691,16 @@ html:lang(he-il) .rtl #wpadminbar *  {
 /* Skip link */
 #wpadminbar .screen-reader-text,
 #wpadminbar .screen-reader-text span {
-	position: absolute;
-	right: -1000em;
-	top: -1000em;
+	border: 0;
+	clip: rect(1px, 1px, 1px, 1px);
+	clip-path: inset(50%);
 	height: 1px;
-	width: 1px;
+	margin: -1px;
 	overflow: hidden;
+	padding: 0;
+	position: absolute;
+	width: 1px;
+	word-wrap: normal !important;
 }
 
 #wpadminbar .screen-reader-shortcut {