events: change status of event.srcElement to legacy

`event.srcElement` is described as legacy in spec.

Refs: https://dom.spec.whatwg.org/#interface-event

Author: deokjinkim

doc/api/events.md

@@ -1852,6 +1852,8 @@ This is not used in Node.js and is provided purely for completeness.
 added: v14.5.0
 -->
 
+> Stability: 3 - Legacy: Use \[`event.target`]\[] instead.
+
 * Type: {EventTarget} The `EventTarget` dispatching the event.
 
 Alias for `event.target`.