Update to net8.0 and MonoGame 3.8.3

[PaulMDemers]

Updated all of the projects to net8.0, updated the MonoGame version to 3.8.3.

There were no real code changes other than fixing a possible null problem in GltfModelFactory.

Demos 1,2 and 3 have been tested.

[vpenades]

Reviewing your changes, I think you missed some important spots (which might ultimetaly prevent the project from being upgrade to net8)

• You didn't update the tools at .config/dotnet-tools.json so you're effectively using the content pipeline and shader compiler of 3.8.1 with 3.8.3 runtimes.
• The reason for this library to exist in the first place and the one that's important to me is the VR Demo. I am not up to date with things, but I recall KNI has been progressively drifting away from MonoGame, and if I recall correctly, one of the breaking changes is that the effects shaders between monogame and KNI are no longer compatible (this would need to be confirmed). So if upgrading MonoGame breaks the VR demo, it's a no go.
• Our clients are still using net6 only, so we do require the project to keep using Net6. That doesn't mean that it can't be lifted to net8... it just means that it needs to support both frameworks. there is no problem using different versions of MonoGame for each framework, so Net6 would use 3.8.1 and net8 would use 3.8.3

The "Pièce de résistance" is the PBR effect shaders, I don't know if they've changed between 3.8.1 and 3.8.3, sometimes they introduce changes that require the shaders to be recompiled. Right now they're being compiled by hand (there's a batch command line to compile them in the PBR project) that's the bit that gives more headaches and what may cause newer versions of KNI to stop working) in the long run it might be required to PBR libraries, one for KNI and one for MonoGame