Improving error message handling and performing assert consolidation

[OsirisTerje]

This issue is a parent for different issues and tasks to be able to improve the handling of error messages, simplify the assert handling, including splitting out the legacy asserts.

This is part of the NUnit 4 planning #3325

Tasks and issues to be done

• ✅ Assert.That methods should autogenerate message, if null #4413
• ✅ Is there any way we could make use of CallerArgumentExpressionAttribute? #3936
• ✅ Issue4413 caller argument expression #4430
• ✅ Add support for native .NET-6.0 target  #4421
• ✅ Move classic asserts into its own project  #4416
• ✅ Remove use of params for messages #4415
• ✅ All Assert messages must be non-null #4435
• ✅ Fix null message in internal static string? ExtendedMessage(string methodName, string? message, string actualExpression, string constraintExpression) #4432