Update States to Bevy 0.10.

[andriyDev]

This basically comes from this Bevy announcement.

States are much simpler now at the cost of some functionality (state stacks).

[andriyDev]

I chose to omit OnUpdate in favour of the run condition in_state. Bevy at HEAD has already removed the OnUpdate system set, since the in_state run condition already does the same thing.

To hopefully make these docs a little more stable for longer, I think using in_state is better.