Replace ReactDOM.render with ReactDOM.hydrate (#9204)

As of React 16, using `ReactDOM.render` in the manner
demonstrated in the `server-render` README will cause a
React deprecation warning. Switching to `ReactDOM.hydrate`
will avoid this. From the React docs:

> Using ReactDOM.render() to hydrate a server-rendered
> container is deprecated and will be removed in React 17.
> Use hydrate() instead.

Source: https://reactjs.org/docs/react-dom.html#render

packages/server-render/README.md

@@ -64,7 +64,7 @@ import { onPageLoad } from "meteor/server-render";
 
 onPageLoad(async sink => {
   const App = (await import("/imports/Client.js")).default;
-  ReactDOM.render(
+  ReactDOM.hydrate(
     <App />,
     document.getElementById("app")
   );
@@ -76,7 +76,7 @@ Note that the `onPageLoad` callback function is allowed to return a
 implemented by an `async` function (as in the client case above).
 
 Note also that the client example does not end up calling any methods of
-the `sink` object, because `ReactDOM.render` has its own similar API. In
+the `sink` object, because `ReactDOM.hydrate` has its own similar API. In
 fact, you are not even required to use the `onPageLoad` API on the client,
 if you have your own ideas about how the client should do its rendering.