Accessibility: Media: Improve the search media field labelling.

Visible `<label>` elements benefit all users. The `placeholder` attribute should not be used as a replacement for visible labels.
Instead, it's supposed to be used only for a short hint to aid users with data entry e.g. a sample value or a brief description of the expected format.

Screen readers may not announce a `placeholder` attribute at all. Other users may suffer from the lack of a visible label and a placeholder used as replacement, for example:

- users with cognitive disabilities may have trouble remembering what the filled field does
- speech recognition users cannot see the name they can speak to set focus on the field
- low-vision users with high text-size may not be able to see the whole placeholder even when it's visible, if its value is clipped by the edge of the input

Props anevins, audrasjb, karmatosed, azaozz, SergeyBiryukov, afercia.
See #40331.
Fixes #47138.


git-svn-id: https://develop.svn.wordpress.org/trunk@46418 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/common.css

@@ -3985,6 +3985,10 @@ img {
 	#screen-meta-links {
 		margin-bottom: 20px; /* Add margins beneath links for better spacing between boxes and elements */
 	}
+
+	.wp-filter .search-form input[type="search"] {
+		font-size: 1rem;
+	}
 }
 
 /* Smartphone */