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/theme.php

@@ -1288,11 +1288,10 @@ function get_header_image_tag( $attr = array() ) {
 	$attr = wp_parse_args(
 		$attr,
 		array(
-			'src'      => $header->url,
-			'width'    => $width,
-			'height'   => $height,
-			'alt'      => $alt,
-			'decoding' => 'async',
+			'src'    => $header->url,
+			'width'  => $width,
+			'height' => $height,
+			'alt'    => $alt,
 		)
 	);