Set rendered_diffs_viewer to default enabled

This feature is ready for production, but as it is part of incubation we will release first as default enabled.

Steps to reproduce

1. On your gdk, open a commit that contains a jupyter notebook (http://gdk.test:3000/gitlab-org/gitlab-test/-/commit/5d6ed1503801ca9dc28e95eeb85a7cf863527aee)
2. By default, the feature flag is on, so it should display the transformed diff
3. Disable the feature flags:

   Feature.disable(:rendered_diff_viewer)

4. Restart gdk
5. Cleaner diff is shown, but the option to select the raw diff is removed