Prefer explicit features for optional deps. (#681)

Implicit features are slated to be removed in a future version
of Rust (2024 edition).

Fixing this exposed 2 instances where the wrong feature was being
checked for `serde` vs `serde-serialize`.

src/pipeline/physics_pipeline.rs

@@ -767,7 +767,7 @@ mod test {
         );
     }
 
-    #[cfg(feature = "serde")]
+    #[cfg(feature = "serde-serialize")]
     #[test]
     fn rigid_body_removal_snapshot_handle_determinism() {
         let mut colliders = ColliderSet::new();