Date/Time: Replace all instances of date() with gmdate().

Use of `date()` in core depends on PHP timezone set to UTC and not changed by third party code (which cannot be guaranteed).

`gmdate()` is functionally equivalent, but is not affected by PHP timezone setting: it's always UTC, which is the exact behavior the core needs.

Props nielsdeblaauw, Rarst.
Fixes #46438. See #44491.

git-svn-id: https://develop.svn.wordpress.org/trunk@45424 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp.php

@@ -443,7 +443,7 @@ class WP {
 			}
 
 			if ( ! $wp_last_modified ) {
-				$wp_last_modified = date( 'D, d M Y H:i:s' );
+				$wp_last_modified = gmdate( 'D, d M Y H:i:s' );
 			}
 
 			$wp_last_modified .= ' GMT';