Remove deprecated ReactDOM methods from demo-app

packages/demo-app/src/index.tsx

@@ -14,22 +14,24 @@
  */
 
 import * as React from "react";
-import * as ReactDOM from "react-dom";
+import * as ReactDOM from "react-dom/client";
 
 import { BlueprintProvider, FocusStyleManager } from "@blueprintjs/core";
 
 import { Examples } from "./examples/Examples";
 
 FocusStyleManager.onlyShowFocusOnTabs();
 
+const container = document.getElementById("blueprint-demo-app");
+const root = ReactDOM.createRoot(container);
+
 (async () => {
     // Wait until CSS is loaded before rendering components because some of them (like Table)
     // rely on those styles to take accurate DOM measurements.
     await import("./index.scss");
-    ReactDOM.render(
+    root.render(
         <BlueprintProvider>
             <Examples />
         </BlueprintProvider>,
-        document.querySelector("#blueprint-demo-app"),
     );
 })();