Remove some old references to CoreSet (#9833)

# Objective

Remove some references to `CoreSet` which was removed in #8079.

crates/bevy_ecs/src/world/mod.rs

@@ -885,8 +885,8 @@ impl World {
     /// of detection to be recorded.
     ///
     /// When using `bevy_ecs` as part of the full Bevy engine, this method is added as a system to the
-    /// main app, to run during `CoreSet::Last`, so you don't need to call it manually. When using
-    /// `bevy_ecs` as a separate standalone crate however, you need to call this manually.
+    /// main app, to run during `Last`, so you don't need to call it manually. When using `bevy_ecs`
+    /// as a separate standalone crate however, you need to call this manually.
     ///
     /// ```
     /// # use bevy_ecs::prelude::*;

examples/ecs/apply_deferred.rs

@@ -108,7 +108,7 @@ fn setup(mut commands: Commands) {
 // has finished during this tick, we spawn a new Apple and a new Orange.
 //
 // The commands that we have added here will normally be flushed by Bevy
-// as part of the `CoreSet::UpdateFlush` set, but because we have ordered
+// after all systems in the schedule have run, but because we have ordered
 // this system to run before `apply_deferred.in_set(CustomFlush)`,
 // these commands added here will be flushed during our custom flush.
 fn despawn_old_and_spawn_new_fruits(

examples/ecs/removal_detection.rs

@@ -11,7 +11,7 @@ fn main() {
     // and the Component` is removed.
     //
     // With these constraints in mind we make sure to place the system that removes a `Component` in
-    // `CoreSet::Update', and the system that reacts on the removal in `CoreSet::PostUpdate`.
+    // `Update', and the system that reacts on the removal in `PostUpdate`.
     App::new()
         .add_plugins(DefaultPlugins)
         .add_systems(Startup, setup)

examples/stress_tests/transform_hierarchy.rs

@@ -185,7 +185,7 @@ fn main() {
         .insert_resource(cfg)
         .add_plugins((MinimalPlugins, TransformPlugin))
         .add_systems(Startup, setup)
-        // Updating transforms *must* be done before `CoreSet::PostUpdate`
+        // Updating transforms *must* be done before `PostUpdate`
         // or the hierarchy will momentarily be in an invalid state.
         .add_systems(Update, update)
         .run();