Remove uses of TransformComponent.ChildEntities (#22442)

* Make output of ChildEnumerator non-nullable

* Remove uses of ChildEntities

* poke tests

---------

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Server/Cargo/Systems/PricingSystem.cs

@@ -342,8 +342,8 @@ public sealed class PricingSystem : EntitySystem
     {
         var xform = Transform(grid);
         var price = 0.0;
-
-        foreach (var child in xform.ChildEntities)
+        var enumerator = xform.ChildEnumerator;
+        while (enumerator.MoveNext(out var child))
         {
             if (predicate is null || predicate(child))
             {