twentysixteen: Center text in image captions

It seems to me that this would be a better default choice. Ideally, we
should be able to change it via the UI, but as far as I know, that's
not possible at the moment. One workaround is to use a centered
paragraph via the code editor, which feels like a hack.

Related: https://core.trac.wordpress.org/ticket/65111

Author: Tamaranch

src/wp-content/themes/twentysixteen/css/blocks.css

@@ -25,11 +25,7 @@ Description: Used to style blocks.
 	font-style: italic;
 	line-height: 1.6153846154;
 	padding-top: 0.5384615385em;
-	text-align: left;
-}
-
-.rtl [class^="wp-block-"] figcaption {
-	text-align: right;
+	text-align: center;
 }
 
 /*--------------------------------------------------------------

src/wp-content/themes/twentysixteen/css/editor-blocks.css

@@ -271,7 +271,7 @@ figure[class*="wp-block-"] > figcaption {
 	font-style: italic;
 	line-height: 1.6153846154;
 	padding-top: 0.5384615385em;
-	text-align: start;
+	text-align: center;
 }
 
 /*--------------------------------------------------------------