Remove soft dependency on Unity3D.SDK

[oliverbooth]

X10D.Unity currently soft-references Unity3D.SDK 2021.1.14.1. This NuGet package has not been updated since 11 July 2021, and as such is not getting any new API introduced by 2022 or 2023, making it impossible to create extension methods for newer types.

One possible solution to this would be to create a dummy assembly in the repository that X10D.Unity soft-references, which mirrors the Unity API by defining the types necessary to extend. However, this adds another layer of maintenance that I'm not entirely prepared to do.

While I can reference the Unity assemblies using my local installation while developing, there also needs to be a way for GitHub Actions to have access to the assemblies so that CD can continue to build the X10D.Unity artifacts. There is potential here, as GameCI offers a Unity Docker image which appears to be maintained and updated regularly.

I might be able to tap into this, and create a workflow which pulls the latest Unity Editor and references those assemblies. However failing this, it may also be the case that we simply have to wait until Unity migrates to SDK-style csproj, wherein I can reference Unity assemblies natively without the hassle of introducing new workflows.

Further research is required, and this issue will be placed on the backburner with tangible no milestone for now.

[oliverbooth]

Unity is planning to upgrade to CoreCLR in the near future, the discussion of which is consistently active and reasonably transparent.

When this time comes, it may be possible to utilise Unity's own libraries as assembly references, or perhaps the X10D.Unity project could be changed to match Unity's new MSBuild integration.

However for the mean time, it may be appropriate to suppress NU1701 to prevent the analyser from the unnecessary warning, since the reference to Unity3D.SDK is a soft dependency, and only referenced privately.

[oliverbooth]

Superseded by #86