Just edit the index.html as this:

<!-- replace this -->
<!-- <script src="flutter_bootstrap.js" async></script> -->

<!-- with this -->
<script>
  {{flutter_js}}
  {{flutter_build_config}}

  _flutter.loader.loadEntrypoint({
    onEntrypointLoaded: async function (engineInitializer) {
      let appRunner = await engineInitializer.initializeEngine({
        // JsFlutterConfiguration goes here...
        canvasKitBaseUrl: "/canvaskit/",
      });
      appRunner.runApp();
    },
  });
</script>

References:

• Flutter web app initialization documentation provides the custom bootstrap script template.

• Source code of flutter engine's configuration indicates that window.flutterConfiguration is now deprecated.