Borrow instead of consuming in EventReader::clear (#6851)

The PR fixes the interface of `EventReader::clear`. Currently, the method consumes the reader, which makes it unusable.

## Changelog

- `EventReader::clear` now takes a mutable reference instead of consuming the event reader. 

## Migration Guide

`EventReader::clear` now takes a mutable reference instead of consuming the event reader. This means that `clear` now needs explicit mutable access to the reader variable, which previously could have been omitted in some cases:

```rust
// Old (0.9)
fn clear_events(reader: EventReader<SomeEvent>) {
  reader.clear();
}

// New (0.10)
fn clear_events(mut reader: EventReader<SomeEvent>) {
  reader.clear();
}
``` 

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_ecs/src/event.rs

@@ -226,7 +226,7 @@ impl<'w, 's, E: Event> EventReader<'w, 's, E> {
     /// #
     /// struct CollisionEvent;
     ///
-    /// fn play_collision_sound(events: EventReader<CollisionEvent>) {
+    /// fn play_collision_sound(mut events: EventReader<CollisionEvent>) {
     ///     if !events.is_empty() {
     ///         events.clear();
     ///         // Play a sound
@@ -246,7 +246,7 @@ impl<'w, 's, E: Event> EventReader<'w, 's, E> {
     /// In those situations you generally want to consume those events to make sure they don't appear in the next frame.
     ///
     /// For more information see [`EventReader::is_empty()`].
-    pub fn clear(mut self) {
+    pub fn clear(&mut self) {
         self.iter().last();
     }
 }
@@ -817,7 +817,7 @@ mod tests {
         events.send(TestEvent { i: 0 });
         world.insert_resource(events);
 
-        let mut reader = IntoSystem::into_system(|events: EventReader<TestEvent>| -> bool {
+        let mut reader = IntoSystem::into_system(|mut events: EventReader<TestEvent>| -> bool {
             if !events.is_empty() {
                 events.clear();
                 false

examples/games/breakout.rs

@@ -404,7 +404,7 @@ fn check_for_collisions(
 }
 
 fn play_collision_sound(
-    collision_events: EventReader<CollisionEvent>,
+    mut collision_events: EventReader<CollisionEvent>,
     audio: Res<Audio>,
     sound: Res<CollisionSound>,
 ) {