KSES: Add opacity to the list of safe CSS properties.

Original PR from Gutenberg repository:
* [WordPress/gutenberg#59891 #59891 Update overlay step function in cover block]

Reference: [https://developer.mozilla.org/en-US/docs/Web/CSS/opacity MDN Web Docs: opacity].

Props sunil25393, wildworks, poena, Mamaduka, presstoke.
Fixes #61536.
Built from https://develop.svn.wordpress.org/trunk@58709


git-svn-id: http://core.svn.wordpress.org/trunk@58111 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/kses.php

@@ -2526,6 +2526,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'list-style-type',
 			'object-fit',
 			'object-position',
+			'opacity',
 			'overflow',
 			'vertical-align',
 			'writing-mode',

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.7-alpha-58708';
+$wp_version = '6.7-alpha-58709';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.