Media: Rely on wp_get_loading_optimization_attributes() to add decoding="async" to images.

The `wp_get_loading_optimization_attributes()` function was introduced in 6.3, as a single centralized place to control loading optimization attributes for various tags, most importantly images.

This changeset consolidates the `decoding="async"` optimization, which was added in 6.1, to occur solely as part of `wp_get_loading_optimization_attributes()`, removing duplicate code and allowing centralized filtering based on [56651].

As part of the change, the `wp_img_tag_add_decoding_attr()` function has been deprecated. The filter of the same name continues to be maintained for backward compatibility, as before covering only images that are part of a content blob such as post content (`the_content`).

Props pereirinha, mukesh27, joemcgill, flixos90.
Fixes #58892.
See #53232.


git-svn-id: https://develop.svn.wordpress.org/trunk@56690 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/pluggable.php

@@ -2845,7 +2845,6 @@ if ( ! function_exists( 'get_avatar' ) ) :
 			'loading'       => null,
 			'fetchpriority' => null,
 			'extra_attr'    => '',
-			'decoding'      => 'async',
 		);
 
 		if ( empty( $args ) ) {