Date/Time: Remove `mysql2date()` usage in `get_feed_build_date()` to ensure the output includes correct timezone offset.

With the changes in [45908], `mysql2date()` works correctly for all local time inputs, but should not be used for UTC time inputs.

Add a unit test.

Props Rarst, lisota.
Merges [46756] and [46757] to the 5.3 branch.
Fixes #48675.
Built from https://develop.svn.wordpress.org/branches/5.3@46774


git-svn-id: http://core.svn.wordpress.org/branches/5.3@46574 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/feed.php

@@ -683,7 +683,13 @@ function get_feed_build_date( $format ) {
 	}
 
 	// Determine the maximum modified time.
-	$max_modified_time = mysql2date( $format, max( $modified_times ), false );
+	$datetime = date_create_immutable_from_format(
+		'Y-m-d H:i:s',
+		max( $modified_times ),
+		new DateTimeZone( 'UTC' )
+	);
+
+	$max_modified_time = $datetime->format( $format );
 
 	/**
 	 * Filters the date the last post or comment in the query was modified.

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3.1-alpha-46773';
+$wp_version = '5.3.1-alpha-46774';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.