Fix window.flutterWebRenderer is now deprecated.

[lockieRichter]

Our web builds are throwing the following warnings. Should be an easy fix.

window.flutterWebRenderer is now deprecated.
Use engineInitializer.initializeEngine(config) instead.
See: https://docs.flutter.dev/development/platform-integration/web/initialization

[behold-com]

Index.html:

`<script>
let useHtml = true;

window.addEventListener('load', function (ev) {
  _flutter.loader.loadEntrypoint({
    serviceWorker: {
      serviceWorkerVersion: serviceWorkerVersion,
    },
    onEntrypointLoaded: function (engineInitializer) {
      let config = {
        renderer: useHtml ? "html" : "canvaskit",
      };
      engineInitializer.initializeEngine(config).then(function (appRunner) {
        appRunner.runApp();
      });
    }
  });
});

</script>`