Feeds: Ensure a feed’s `<lastBuildDate>` has the correct date.

The maximum modified time for a is currently being determined after an arbitrary date format is applied. Because the string sort in `max()` is being used to return the highest (most recent) modified value, the incorrect date is sometimes returned. This was introduced in [45247].

Props onlanka, Rarsr, mukesh27.
Fixes #47304.

git-svn-id: https://develop.svn.wordpress.org/trunk@45367 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/feed.php

@@ -672,14 +672,7 @@ function get_feed_build_date( $format ) {
 	}
 
 	// Determine the maximum modified time.
-	$max_modified_time = max(
-		array_map(
-			function ( $time ) use ( $format ) {
-				return mysql2date( $format, $time, false );
-			},
-			$modified_times
-		)
-	);
+	$max_modified_time = mysql2date( $format, max( $modified_times ), false );
 
 	/**
 	 * Filters the date the last post or comment in the query was modified.