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.


git-svn-id: https://develop.svn.wordpress.org/trunk@41622 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/install.css

@@ -402,14 +402,16 @@ body.language-chooser {
 
 .screen-reader-input,
 .screen-reader-text {
-	position: absolute;
-	margin: -1px;
-	padding: 0;
-	height: 1px;
-	width: 1px;
-	overflow: hidden;
-	clip: rect(0 0 0 0);
 	border: 0;
+	clip: rect(1px, 1px, 1px, 1px);
+	clip-path: inset(50%);
+	height: 1px;
+	margin: -1px;
+	overflow: hidden;
+	padding: 0;
+	position: absolute;
+	width: 1px;
+	word-wrap: normal !important;
 }
 
 .spinner {