Cleanup: Deprecate the Never type in favor of `!` once stabilized

The never type is currently a type alias for `Infallible`, an enum of no variants.
It does its job except it doesn't implement `Into for T`, which is technically possible using `unreachable!()` since if it can't be constructed, there's nothing to convert.

See https://github.com/rust-lang/rust/issues/35121

#### Solution

- Mark `Never` as deprecated
- Update related documentation and mentions of `Never` to instead refer to `!`
- Update or deprecate the `map_never` operator if `map_into` can take its place.

#### Related Issues:

- https://github.com/AlexAegis/rx_bevy/issues/26