Require read-only queries in QueryState::par_iter (#8832)

# Objective

The method `QueryState::par_iter` does not currently force the query to
be read-only. This means you can unsoundly mutate a world through an
immutable reference in safe code.

```rust
fn bad_system(world: &World, mut query: Local<QueryState<&mut T>>) {
    query.par_iter(world).for_each_mut(|mut x| *x = unsoundness);
}
```

## Solution

Use read-only versions of the `WorldQuery` types.

---

## Migration Guide

The function `QueryState::par_iter` now forces any world accesses to be
read-only, similar to how `QueryState::iter` works. Any code that
previously mutated the world using this method was *unsound*. If you
need to mutate the world, use `par_iter_mut` instead.

crates/bevy_ecs/src/query/state.rs

@@ -865,11 +865,14 @@ impl<Q: WorldQuery, F: ReadOnlyWorldQuery> QueryState<Q, F> {
     ///
     /// [`par_iter_mut`]: Self::par_iter_mut
     #[inline]
-    pub fn par_iter<'w, 's>(&'s mut self, world: &'w World) -> QueryParIter<'w, 's, Q, F> {
+    pub fn par_iter<'w, 's>(
+        &'s mut self,
+        world: &'w World,
+    ) -> QueryParIter<'w, 's, Q::ReadOnly, F::ReadOnly> {
         self.update_archetypes(world);
         QueryParIter {
             world,
-            state: self,
+            state: self.as_readonly(),
             last_run: world.last_change_tick(),
             this_run: world.read_change_tick(),
             batching_strategy: BatchingStrategy::new(),