Fix DetectChanges::last_changed returning the wrong tick (#7560)

# Objective

Make `last_changed` behave as described in its docs.

## Solution

- Return `changed` instead of `last_change_tick`. `last_change_tick` is the system's previous tick and is just used for comparison.
- Update the docs of the similarly named `set_last_changed` (which does correctly interact with `last_change_tick`) to clarify that the two functions touch different data. (I advocate for renaming one or the other if anyone has any good suggestions).

It also might make sense to return a cloned `Tick` instead of `u32`.

---

## Changelog

- Fixed `DetectChanges::last_changed` returning the wrong value.
- Fixed `DetectChangesMut::set_last_changed` not actually updating the `changed` tick. 

## Migration Guide

- The incorrect value that was being returned by `DetectChanges::last_changed` was the previous run tick of the system checking for changed values. If you depended on this value, you can get it from the `SystemChangeTick` `SystemParam` instead.

crates/bevy_ecs/src/change_detection.rs

@@ -49,7 +49,7 @@ pub trait DetectChanges {
     /// Returns `true` if this value was added or mutably dereferenced after the system last ran.
     fn is_changed(&self) -> bool;
 
-    /// Returns the change tick recording the previous time this data was changed.
+    /// Returns the change tick recording the time this data was most recently changed.
     ///
     /// Note that components and resources are also marked as changed upon insertion.
     ///
@@ -103,7 +103,7 @@ pub trait DetectChangesMut: DetectChanges {
     /// **Note**: This operation cannot be undone.
     fn set_changed(&mut self);
 
-    /// Manually sets the change tick recording the previous time this data was mutated.
+    /// Manually sets the change tick recording the time when this data was last mutated.
     ///
     /// # Warning
     /// This is a complex and error-prone operation, primarily intended for use with rollback networking strategies.
@@ -150,7 +150,7 @@ macro_rules! change_detection_impl {
 
             #[inline]
             fn last_changed(&self) -> u32 {
-                self.ticks.last_change_tick
+                self.ticks.changed.tick
             }
         }
 
@@ -186,7 +186,9 @@ macro_rules! change_detection_mut_impl {
 
             #[inline]
             fn set_last_changed(&mut self, last_change_tick: u32) {
-                self.ticks.last_change_tick = last_change_tick
+                self.ticks
+                    .changed
+                    .set_changed(last_change_tick);
             }
 
             #[inline]