Fix of React SyntheticEvent.timeStamp property type (#15165)

The `timeStamp` property of events is a numeric value in milliseconds, not a `Date`.
https://developer.mozilla.org/en-US/docs/Web/API/Event/timeStamp

react/index.d.ts

@@ -289,7 +289,7 @@ declare namespace React {
         persist(): void;
         // If you thought this should be `EventTarget & T`, see https://github.com/DefinitelyTyped/DefinitelyTyped/pull/12239
         target: EventTarget;
-        timeStamp: Date;
+        timeStamp: number;
         type: string;
     }