Fix warnings from integration tests with chromedriver

Currently, we have the follow warnings in tests:
```
Warning: In index.html:53: Local variable for "serviceWorkerVersion" is deprecated. Use "{{flutter_service_worker_version}}" template token instead. See https://docs.flutter.dev/platform-integration/web/initialization for more details.
Warning: In index.html:62: "FlutterLoader.loadEntrypoint" is deprecated. Use "FlutterLoader.load" instead. See https://docs.flutter.dev/platform-integration/web/initialization for more details.
```

I'm guessing this is something to do with `example/web/index.html`:
https://github.com/google/webcrypto.dart/blob/master/example/web/index.html

Perhaps we should try to create a new template and copy/cargo-cult the setup from there.

It's unclear if this will fix chromedriver tests on macos.