fix: Replaced HashSet.UnionWith() with loop in NetworkBehaviourUpdate() to avoid heap alloc (#3568)

Discovered while profiling a server that had frequent pending dirty
network objects to process

## Changelog

Fixed: Removed heap alloc in NetworkBehaviourUpdate

## Testing and Documentation

- No tests have been added.
- No documentation changes or additions were necessary.

## Backport
Requires up-port

---------

Co-authored-by: Emma <[email protected]>

Author: louismclaughlin

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -14,6 +14,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Removed allocation to the heap in NetworkBehaviourUpdate. (#3568)
 - Fixed issue where NetworkConfig.ConnectionData could cause the ConnectionRequestMessage to exceed the transport's MTU size and would result in a buffer overflow error. (#3565)
 
 ### Changed

com.unity.netcode.gameobjects/Runtime/Core/NetworkBehaviourUpdater.cs

@@ -34,7 +34,10 @@ internal void NetworkBehaviourUpdate(bool forceSend = false)
 #endif
             try
             {
-                m_DirtyNetworkObjects.UnionWith(m_PendingDirtyNetworkObjects);
+                foreach (var o in m_PendingDirtyNetworkObjects)
+                {
+                    m_DirtyNetworkObjects.Add(o);
+                }
                 m_PendingDirtyNetworkObjects.Clear();
 
                 // NetworkObject references can become null, when hidden or despawned. Once NUll, there is no point