fix: NetworkObject.GetNetworkBehaviourAtOrderIndex permission level (#3541)

This resolves a regression bug where
NetworkObject.GetNetworkBehaviourAtOrderIndex was converted from public
to internal during the original v2.0.0-pre branch creation.

MTTB-1427
fix: #3528

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## Changelog

- Fixed: Regression issue in v2.x where
`NetworkObject.GetNetworkBehaviourAtOrderIndex` was converted from
public to internal.

## Testing and Documentation

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

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## Backport
This issue is specific to v2.x.

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Author: NoelStephensUnity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -16,6 +16,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed regression issue in v2.x where `NetworkObject.GetNetworkBehaviourAtOrderIndex` was converted from public to internal. (#3541)
 - Fixed ensuring OnValueChanged callback is still triggered on the authority when a collection changes and then reverts to the previous value in the same frame. (#3539)
 - Fixed synchronizing the destroyGameObject parameter to clients for InScenePlaced network objects. (#3514)
 - Fixed distributed authority related issue where enabling the `NetworkObject.DestroyWithScene` would cause errors when a destroying non-authority instances due to loading (single mode) or unloading scene events. (#3500)

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

@@ -2762,7 +2762,12 @@ public ushort GetNetworkBehaviourOrderIndex(NetworkBehaviour instance)
             return 0;
         }
 
-        internal NetworkBehaviour GetNetworkBehaviourAtOrderIndex(ushort index)
+        /// <summary>
+        /// Returns the <see cref="NetworkBehaviour"/> at the ordered index value which can be obtained using <see cref="GetNetworkBehaviourOrderIndex"/>.
+        /// </summary>
+        /// <param name="index">The order index value of the <see cref="NetworkBehaviour"/>.</param>
+        /// <returns>The <see cref="NetworkBehaviour"/> at the ordered index value or null if it does not exist.</returns>
+        public NetworkBehaviour GetNetworkBehaviourAtOrderIndex(ushort index)
         {
             if (index >= ChildNetworkBehaviours.Count)
             {