replacer: change timezone to UTC for "time.now.http" placeholders (#5774)

Author: WeidiDeng

replacer.go

@@ -319,7 +319,10 @@ func globalDefaultReplacements(key string) (any, bool) {
 	case "time.now":
 		return nowFunc(), true
 	case "time.now.http":
-		return nowFunc().Format(http.TimeFormat), true
+		// According to the comment for http.TimeFormat, the timezone must be in UTC
+		// to generate the correct format.
+		// https://github.com/caddyserver/caddy/issues/5773
+		return nowFunc().UTC().Format(http.TimeFormat), true
 	case "time.now.common_log":
 		return nowFunc().Format("02/Jan/2006:15:04:05 -0700"), true
 	case "time.now.year":