Use bodyClasses option to register static classes on skin

Bug: T298734
Change-Id: Ia2ae8e1c655aff8783c2602da992632cd5877bf8

includes/Hooks.php

@@ -606,10 +606,6 @@ class Hooks {
 		// list.
 		if ( self::isSkinVersionLegacy( $skinName ) ) {
 			$bodyAttrs['class'] .= ' skin-vector-legacy';
-		} else {
-			// The modern Vector skin must also carry skin-vector for compatibility with older
-			// skins.
-			$bodyAttrs['class'] .= ' skin-vector skin-vector-search-vue';
 		}
 
 		$config = $sk->getConfig();

skin.json

@@ -36,6 +36,10 @@
 							"tag": "span"
 						}
 					},
+					"bodyClasses": [
+						"skin-vector",
+						"skin-vector-search-vue"
+					],
 					"messages": [
 						"tooltip-p-logo",
 						"vector-opt-out-tooltip",