the DoesNotReturnAttribute should be enabled for NETCOREAPP3_0_OR_GREATER

Currently, the generated code has the following method.

```cs
#if NETSTANDARD2_1_OR_GREATER
        [DoesNotReturn]
#endif
        private static TV ThrowInvalidCastException<TV>() => throw new InvalidCastException();
```

When targeting f.e. .NET7 the attribute is disabled. It'd be nice to have it enabled when `NETCOREAPP3_0_OR_GREATER` is set.