Themes: Reduce usage of wp_get_theme function.

Calling the `wp_get_theme` function creates a instance of the `WP_Theme` class. This can be a performance issue, if all you need is one property of the class instance. This change replaces the usage of `wp_get_theme()->get_stylesheet()` with `get_stylesheet()` to improve performance.

Props spacedmonkey, flixos90, peterwilsoncc, desrosj.
Fixes #57057.

git-svn-id: https://develop.svn.wordpress.org/trunk@54817 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-template.php

@@ -145,7 +145,7 @@ function resolve_block_template( $template_type, $template_hierarchy, $fallback_
 
 	// Find all potential templates 'wp_template' post matching the hierarchy.
 	$query     = array(
-		'theme'    => wp_get_theme()->get_stylesheet(),
+		'theme'    => get_stylesheet(),
 		'slug__in' => $slugs,
 	);
 	$templates = get_block_templates( $query );