Improve handling of centered 1-col galleries with small images (#11040)

* Improve handling of centered 1-col galleries with small images

If you insert a gallery that has a single column, and multiple smallish images, then center it, the images inside do not appear centered.

This is technically correct, since the _gallery itself_ is centered, but because it's full width and because the images might vary in widths, this doesn't appear to make any difference to the content inside. So although "correct", it's not a good user experience.

This PR simply centers the content also.

* Remove editor specific alignments.

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

@@ -133,8 +133,6 @@
 	}
 
 	// Apply max-width to floated items that have no intrinsic width.
-	[data-align="left"] &,
-	[data-align="right"] &,
 	&.alignleft,
 	&.alignright {
 		max-width: $content-width / 2;
@@ -148,4 +146,11 @@
 	&.alignright {
 		display: flex;
 	}
+
+	// If the gallery is centered, center the content inside as well.
+	&.aligncenter {
+		.blocks-gallery-item figure {
+			justify-content: center;
+		}
+	}
 }