Castle.Core modernization

[Romfos]

I think time is come to drop legacy platforms

Changed:

• Now target frameworks are: .NET 8, .NET Framework 4.6.2, .NET Standard 2.0
• Tests platforms now are: .NET Framework 4.6.2, .NET 8, .NET 9
• Lang version now is 13
• Dependencies were updated
• Github action are updated
• Update Explicit.NuGet.Versions

Removed:

• .NET Core 2.1, .NET Core 3.1, .NET 6, and mono tests
• .NET Standard 2.1 tfm

links:

• .NET Core 2.1 EOL: https://devblogs.microsoft.com/dotnet/net-core-2-1-will-reach-end-of-support-on-august-21-2021/
• .NET Core 3.1 EOL: https://devblogs.microsoft.com/dotnet/net-core-3-1-will-reach-end-of-support-on-december-13-2022/
• .NET 6 EOL: https://devblogs.microsoft.com/dotnet/dotnet-6-end-of-support/
• Microsoft no longer support mono, more info: https://github.com/mono/mono

Technically .NET Standard 2.0 version is still here and you can install it in old .NET 6\2.1\3.1\mono\e.t.c projects, but we:

• Don't test changes
• Don't provide any support
• Don't do any optimizations
• And don't recommend to use Castle.Core on these EOL platforms

related: #683

[Romfos]

Do we need appveyor if we have github actions?