refactor: Optimize editor assets endpoint URL processing (#45358)

* refactor: Optimize editor assets endpoint URL processing

Prefer the `str_starts_with` utility since it is available in PHP 8 and
in a polyfill for WordPress 5.9.0 onward. Jetpack requires minimum of
WordPress 6.7.

- https://make.wordpress.org/core/2022/01/10/wordpress-5-9-and-php-8-0-8-1/
- https://github.com/Automattic/jetpack/blob/a5b0d2eb5fef8a5c59c95d0972c812ef2c6b994a/projects/plugins/jetpack/jetpack.php#L43

* changelog

Author: dcalhoun

projects/plugins/jetpack/_inc/lib/core-api/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-block-editor-assets.php

@@ -380,7 +380,7 @@ private function is_allowed_plugin_handle( $handle ) {
 	 * @return string The absolute URL.
 	 */
 	public function make_url_absolute( $src ) {
-		if ( ! empty( $src ) && strpos( $src, '/' ) === 0 && strpos( $src, '//' ) !== 0 ) {
+		if ( ! empty( $src ) && str_starts_with( $src, '/' ) && ! str_starts_with( $src, '//' ) ) {
 			return site_url( $src );
 		}
 		return $src;

projects/plugins/jetpack/changelog/refactor-optimize-editor-assets-url-processing

@@ -0,0 +1,4 @@
+Significance: patch
+Type: other
+
+Editor assets endpoint: optimize URL processing