Date/Time: Correct the time format in `get_feed_build_date()` to use 24-hour format.

Follow-up to [46756].

See #48675.

git-svn-id: https://develop.svn.wordpress.org/trunk@46757 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/feed.php

@@ -683,7 +683,7 @@ function get_feed_build_date( $format ) {
 
 	// Determine the maximum modified time.
 	$datetime = date_create_immutable_from_format(
-		'Y-m-d h:i:s',
+		'Y-m-d H:i:s',
 		max( $modified_times ),
 		new DateTimeZone( 'UTC' )
 	);