chore: 6000.3.0a6 updates (#3600)

## Purpose of this PR
Adding asmdef define for 6000.3.0a6 or higher and using that within
RigidbodyContactEventManager to switch from using otherBodyInstanceID to
otherBodyEntityId.

### Jira ticket
NA

### Changelog
NA

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## Documentation
[//]: # (
This section is REQUIRED and should mention what documentation changes
were following the changes in this PR.
We should always evaluate if the changes in this PR require any
documentation changes.
)

- No documentation changes or additions were necessary.


## Testing & QA
[//]: #  (
This section is REQUIRED and should describe how the changes were tested
and how should they be tested when Playtesting for the release.
It can range from "edge case covered by unit tests" to "manual testing
required and new sample was added".
Expectation is that PR creator does some manual testing and provides a
summary of it here.)

### Functional Testing
[//]: # (If checked, List manual tests that have been performed.)
_Manual testing :_
- [ ] `Manual testing done`

_Automated tests:_
- [X] `Covered by existing automated tests`
- [ ] `Covered by new automated tests`

_Does the change require QA team to:_

- [ ] `Review automated tests`?
- [ ] `Execute manual tests`?

If any boxes above are checked, please add QA as a PR reviewer.

## Backport
This is an NGO v2.x.x only update since NGO v1.x.x will not be supported
on 6.3.

Author: NoelStephensUnity

com.unity.netcode.gameobjects/Runtime/Components/RigidbodyContactEventManager.cs

@@ -374,8 +374,13 @@ public void Execute(int index)
 
                 var result = new JobResultStruct()
                 {
+#if UNITY_6000_3_0A6_OR_HIGHER
+                    ThisInstanceID = PairedHeaders[index].bodyEntityId,
+                    OtherInstanceID = PairedHeaders[index].otherBodyEntityId,
+#else
                     ThisInstanceID = PairedHeaders[index].bodyInstanceID,
                     OtherInstanceID = PairedHeaders[index].otherBodyInstanceID,
+#endif
                     AverageNormal = averageNormal,
                     HasCollisionStay = collisionStaycount != 0,
                     AverageCollisionStayNormal = averageCollisionStay,

com.unity.netcode.gameobjects/Runtime/Unity.Netcode.Runtime.asmdef

@@ -77,6 +77,11 @@
             "name": "Unity",
             "expression": "6000.1.0a1",
             "define": "HOSTNAME_RESOLUTION_AVAILABLE"
+        },
+        {
+            "name": "Unity",
+            "expression": "6000.3.0a6",
+            "define": "UNITY_6000_3_0A6_OR_HIGHER"
         }
     ],
     "noEngineReferences": false