Invalidate the page pattern cache when the site lang changes

The page pattern cache was keyed on the account language. However the
account language isn't used when making the request to get the page
pattern data. Switching to the site's language (i.e. the same locale
slug used when fetching pattern data) means the cache will be
invalidated correctly.

apps/editing-toolkit/editing-toolkit-plugin/starter-page-templates/class-starter-page-templates.php

@@ -36,7 +36,7 @@ private function __construct() {
 				'starter_page_templates',
 				A8C_ETK_PLUGIN_VERSION,
 				get_option( 'site_vertical', 'default' ),
-				get_locale(),
+				$this->get_verticals_locale(),
 			)
 		);