Unity 6 Fixes

[nnra6864]

This PR aims to fix errors and warnings within Unity 6(LTS) while staying compatible with previous versions.

[nnra6864]

This is mostly complete and ready for review, except for a single line of code.
I couldn't figure out what's the correct replacement for:

get { return XRDevice.GetNativePtr() != System.IntPtr.Zero; }

Osmal from the Unity discord server guessed this is supposed to be a simple null check for the xr device, so I wrote this with the help of Unity docs:

get { return UnityEngine.XR.InputDevices.GetDeviceAtXRNode(XRNode.Head) != null; }

But I have a feeling that's not the same as the code above.

References to docs I used:
https://docs.unity3d.com/6000.3/Documentation/ScriptReference/UnityEngine.XRModule.html
https://docs.unity3d.com/6000.3/Documentation/ScriptReference/XR.XRNode.html

[nnra6864]

There are a few other potential replacements I found in docs:
https://docs.unity3d.com/6000.3/Documentation/ScriptReference/XR.XRSettings-isDeviceActive.html
https://docs.unity3d.com/6000.3/Documentation/ScriptReference/XR.CommonUsages-userPresence.html