Add z-index to fig caption to make sure it is still selectable over a…

…bsolute positioned border. (#28992)

#27312 introduced some absolute positioning to the gallery image border to fix some layout issues, but because this had a z-index specified it was sitting over the top of the caption which could no longer be selected.

This PR just adds a z-index to caption also so it sits over the border.

Co-authored-by: Glen Davies <glen.davies@a8c.com>

packages/block-library/src/gallery/editor.scss

@@ -51,6 +51,9 @@ figure.wp-block-gallery {
 			left: 0;
 			z-index: 1;
 		}
+		figcaption {
+			z-index: 2;
+		}
 	}
 
 	figure.is-transient img {