Mark OpenXRCompositionLayer and its children as experimental

[dsnopek]

This is a follow-up to #89880

I had always intended to mark OpenXRCompositionLayer and its children as experimental, but I guess I forgot to do it in the original PR.

These nodes are new and we don't have much experience with them, so I think they should be experimental in Godot 4.3, in case we find some fundamental issue with them after developer start using them in practice. In a minor version or two, we can remove the experimental flag.

[m4gr3d]

Agree on the approach!

Are we doing the same for the XR nodes introduced in 4.3? If not, I think we should standardize on that approach to give us the flexibility to make changes if needed.

[dsnopek]

Are we doing the same for the XR nodes introduced in 4.3?

I assume you're referring to XRHandModifier3D, XRBodyModifier3D and XRFaceModifier3D?

Back on the PRs that added those:

• We decided not to mark XRHandModifier3D as experimental. Since we were deprecating OpenXRHand in favor of XRHandModifier3D, it would be weird if it was experimental. Also, at the time, we felt like XRHandModifier3D was just a distilled version of OpenXRHand, which we have a lot of experience with and felt confident about. However, his may no longer be the case given the XRSkeletonModifier3D changes, so it may make sense to re-evaluate
• We did mark XRBodyModifier3D and XRFaceModifier3D as experimental

[akien-mga]

Thanks!