[in_app_purchase] Remove cruft to support old versions of Flutter

There is some cruft within the packages of this plugin (in examples and tests) relating to older versions of Flutter. There are a couple of:
```
// TODO(goderbauer): Make this const when that's available on stable.
```

and also a
```
// TODO(asashour): Remove the `ignore` when Dart 3 reaches stable.
```

Both should be fixable now.