Move runtime upgrade to `frame-executive`

Instead of storing the runtime upgraded in the space of `frame-system`,
this moves it to `frame-executive`. We also start storing the
`spec_version` and `impl_version` the last runtime upgrade was applied.

This scheme has multiple advantages:
- We don't need to make sure that runtime upgrade was set.
- This will work in the future when we want to test a runtime upgrade,
because the `on_runtime_upgrade` will be executed automatically when the
runtime changes.