Remove relics of unflattened / flattened stream

[clux]

Would you like to work on this feature?

None

What problem are you trying to solve?

Since 0.92 and in particular #1504, there is no longer any vectors being passed as watcher::Event, so there should be no need to talk about flattening, or have behaviour that references this.

Work already done:

• Deprecate Event::into_iter* helpers

Describe the solution you'd like

1. Rename EventFlatten and references and turn it into an EventFilter (since that's all it does now)
2. Clean up any other relevant references

Describe alternatives you've considered

Remove EventFlatten entirely, but having WatchStreamExt::touched_objects() and WatchStreamExt::applied_objects() go through a filter rather than a flattener seems fine to me. The method does not necessarily imply that it's flattening.

Documentation, Adoption, Migration Strategy

Need to change streams doc in kube.rs to not mention it (i can do that after)

Target crate for feature

kube-runtime