Updating repo for transfer to the .NET Foundation

This updates the repo so it can finish transfer to the .NET Foundation: https://github.com/dotnet-foundation/projects/issues/184

In particular, now that the project agreement has been signed, this:

Updates the copyright to be .NET Foundation and Contributors rather than Microsoft and Contributors
Updates the license from NCSA to MIT
- All prior contributions to LLVMSharp were made under the Microsoft CLA
- NCSA was originally chosen to align with LLVM
- LLVM have themselves since changed to Apache 2.0 License with LLVM exceptions
- The libLLVM nuget package is remaining Apache 2.0 License with LLVM exceptions since that is a binary produced by LLVM themselves
Updates the code of conduct to be the .NET Foundation Code of Conduct rather than the Microsoft Open Source Code of Conduct

URLs and CI are still microsoft/ClangSharp and will need to be updated after the repo moves out of the microsoft GitHub organization.

dotnet / LLVMSharp