Luminance: Fix display of featured images (#8023)

The featured images in Luminance were completely misaligned due to
changes in how Gutenberg has changed the specifity of style rules.
Rather than the custom block-specific CSS for the core/featured-image
block taking priority, the default block library styles did.

The problem has been reported upstream
(https://github.com/WordPress/gutenberg/issues/64453) but in the
meantime, using the theme-wide custom CSS fixes the problem.

luminance/theme.json

@@ -507,9 +507,6 @@
 			"core/post-excerpt": {
 				"css": ".wp-block-post-excerpt .wp-block-post-excerpt__excerpt{line-height: inherit;}"
 			},
-			"core/post-featured-image": {
-				"css": ".wp-block-post-featured-image img{vertical-align:top;width: auto;}"
-			},
 			"core/post-navigation-link": {
 				"typography": {
 					"fontSize": "var(--wp--preset--font-size--small)",
@@ -794,7 +791,8 @@
 			"fontFamily": "var(--wp--preset--font-family--instrument-sans)",
 			"fontSize": "var(--wp--preset--font-size--medium)",
 			"lineHeight": "1.388888889"
-		}
+		},
+		"css": ".wp-block-post-featured-image img{vertical-align:top;width: auto;}"
 	},
 	"templateParts": [
 		{