Adds support for threading lock in .NET 9 and C# 13

[michaelstonis]

Adds support using System.Threading.Lock on platforms that support .NET 9 and C# 13. All unit tests continue to pass.

[neuecc]

I think use polyfiil is better aproach rather than #if every fields.

#if !NET9_0_OR_GREATER

namespace System.Threading
{
    internal sealed class Lock
    {
    }
}

#endif

[michaelstonis]

Ha. Yea, it certainly is. It didn't even cross my mind. lol. I will update this.

Are you good with the concept?

[neuecc]

Yes. That's good.