Bundled Themes: Make Twenty Twenty-One the default theme.

After being imported in [49216], Twenty Twenty-One can now be set as the default theme in WordPress.

See #48110.

git-svn-id: https://develop.svn.wordpress.org/trunk@49220 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/theme.php

@@ -19,6 +19,7 @@ class Tests_Theme extends WP_UnitTestCase {
 		'twentyseventeen',
 		'twentynineteen',
 		'twentytwenty',
+		'twentytwentyone',
 	);
 
 	function setUp() {