Code Modernization: Use str_contains() in get_shortcode_tags_in_content().

`str_contains()` was introduced in PHP 8.0 to perform a case-sensitive check indicating if the string to search in (haystack) contains the given substring (needle).

WordPress core includes a polyfill for `str_contains()` on PHP < 8.0 as of WordPress 5.9.

Follow-up to [52039], [55988], [56245], [56838], [60900].

See #63168.

git-svn-id: https://develop.svn.wordpress.org/trunk@60901 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/shortcodes.php

@@ -182,7 +182,7 @@ function has_shortcode( $content, $tag ) {
  * @return string[] An array of registered shortcode names found in the content.
  */
 function get_shortcode_tags_in_content( $content ) {
-	if ( false === strpos( $content, '[' ) ) {
+	if ( ! str_contains( $content, '[' ) ) {
 		return array();
 	}