Provide updated annotations to Unity

After every release, we should check for any changes to the [JetBrains annotations](https://github.com/JetBrains/JetBrains.Annotations), and pass these changes along to Unity, so they can include them in their next release. Currently, the annotations are out of date, without an easy way for a customer to update (e.g. the `ImplicitUseTargetFlags.WithInheritors` enum value isn't defined in Unity, but is understood by the ReSharper engine).

Once completing this, add a task to do the same again at the next release.