Use AnyCPU as solution platform to support other runtimes than x64

[mdabros]

This PR adds the following:

• Use AnyCPU as solution platform to support other runtimes than x64.
• Update readme with information about osx-arm64 package.

[mdabros]

I have been unable to get the CI running when including the libxgboost-2.0.3-osx-arm64 package in the test project. It works separately, but together with the other packages it seems to cause some conflicts, and some of the other (seems random) os/runtimes are not able to correctly locate the native xgboost lib.

See this build for a successful run using only macos-latest (arm64) and the libxgboost-2.0.3-osx-arm64 package:
https://github.com/mdabros/XGBoostSharp/actions/runs/12311697730

See this build for failure when using all packages:
https://github.com/mdabros/XGBoostSharp/actions/runs/12313512046/job/34367692875

Therefor CI only tests for the other package and runtime combinations. Will see if I can fix the CI issue for osx-arm64 in a later PR.