Fix some warnings shown in nightly (#10012)

# Objective

Fix warnings:
- #[warn(clippy::needless_pass_by_ref_mut)]
- #[warn(elided_lifetimes_in_associated_constant)]

## Solution

- Remove mut
- add &'static

## Errors

```rust
warning: this argument is a mutable reference, but not used mutably
   --> crates/bevy_hierarchy/src/child_builder.rs:672:31
    |
672 |     fn assert_children(world: &mut World, parent: Entity, children: Option<&[Entity]>) {
    |                               ^^^^^^^^^^ help: consider changing to: `&World`
    |
    = note: this is cfg-gated and may require further changes
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
    = note: `#[warn(clippy::needless_pass_by_ref_mut)]` on by default
```


```rust
warning: `&` without an explicit lifetime name cannot be used here
   --> examples/shader/post_processing.rs:120:21
    |
120 |     pub const NAME: &str = "post_process";
    |                     ^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #115010 <rust-lang/rust#115010>
    = note: `#[warn(elided_lifetimes_in_associated_constant)]` on by default
help: use the `'static` lifetime
    |
120 |     pub const NAME: &'static str = "post_process";
    |                      +++++++


```

crates/bevy_hierarchy/src/child_builder.rs

@@ -664,12 +664,12 @@ mod tests {
  };
 
  /// Assert the (non)existence and state of the child's [`Parent`] component.
- fn assert_parent(world: &mut World, child: Entity, parent: Option<Entity>) {
+ fn assert_parent(world: &World, child: Entity, parent: Option<Entity>) {
  assert_eq!(world.get::<Parent>(child).map(|p| p.get()), parent);
  }
 
  /// Assert the (non)existence and state of the parent's [`Children`] component.
- fn assert_children(world: &mut World, parent: Entity, children: Option<&[Entity]>) {
+ fn assert_children(world: &World, parent: Entity, children: Option<&[Entity]>) {
  assert_eq!(world.get::<Children>(parent).map(|c| &**c), children);
  }
 

examples/shader/post_processing.rs

@@ -117,7 +117,7 @@ impl Plugin for PostProcessPlugin {
 #[derive(Default)]
 struct PostProcessNode;
 impl PostProcessNode {
- pub const NAME: &str = "post_process";
+ pub const NAME: &'static str = "post_process";
 }
 
 // The ViewNode trait is required by the ViewNodeRunner