Swap to using EventWriter and EventReader in internal engine code

[alice-i-cecile]

Many of our internal systems were written before the advent of EventWriter and EventReader, and so work with either ResMut<Events<T>> or Res<Events<T>>.

These can (and probably should) generally be swapped to EventWriter and EventReader because:

1. The intent is clearer, making the code easier to later modify and read.
2. The API is smaller, making it harder for subtle bugs to be introduced.
3. Any later optimizations we might perform will rely on EventReader and EventWriter, rather than the raw resource types.

You can see an example here:

bevy/crates/bevy_gilrs/src/gilrs_system.rs

Line 19 in e48f9d8

pub fn gilrs_event_system(

Finding all of these should be easy, just search for Res<Events and ResMut<Events in the code base. Fixing them should be similarly straightforward, although please leave a comment in your PR if you found any usages that could not be trivially converted.

[PaperCow]

I wasn't actually able to find any other references to Res<Events or ResMut<Events in the code base outside of the given example. It appears they were mostly removed in #1244 and #1575 but the example in this issue was added after these PRs.

Assuming I'm not just missing some other references, I made a PR #3413 to clean up this one instance.