Code Modernization: Replace usage of strpos() with str_starts_with().

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

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

This commit replaces `0 === strpos( ... )` with `str_starts_with()` in core files, making the code more readable and consistent, as well as improving performance.

While `strpos()` is slightly faster than the polyfill on PHP < 8.0, `str_starts_with()` is noticeably faster on PHP 8.0+, as it is optimized to avoid unnecessarily searching along the whole haystack if it does not find the needle.

Follow-up to [52039], [52040], [52326].

Props spacedmonkey, costdev, sabernhardt, mukesh27, desrosj, jorbin, TobiasBg, ayeshrajans, lgadzhev, SergeyBiryukov.
Fixes #58012.

git-svn-id: https://develop.svn.wordpress.org/trunk@55703 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/widgets/class-wp-widget-block.php

@@ -223,7 +223,7 @@ class WP_Widget_Block extends WP_Widget {
 	 * @return bool Updated `is_wide` value.
 	 */
 	public function set_is_wide_widget_in_customizer( $is_wide, $widget_id ) {
-		if ( strpos( $widget_id, 'block-' ) === 0 ) {
+		if ( str_starts_with( $widget_id, 'block-' ) ) {
 			return false;
 		}