Rename `LifetimeAnnotationAttribute` to `ScopedRefAttribute` and remove unnecessary members

[cston]

The LifetimeAnnotationAttribute synthesized by the compiler for scoped should be renamed to align with the corresponding runtime attribute for unscoped (see dotnet/runtime#72074).

And the IsRefScoped and IsValueScoped properties are unnecessary and can be removed because that can be determined based on whether the parameter is passed by reference or by value.

The updated synthesized attribute API:

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
    internal sealed class ScopedRefAttribute : Attribute
    {
    }
}

Spec change: dotnet/csharplang#6325

[cston]

cc @jaredpar, @jcouv

[Sergio0694]

Nit: I assume the name should be [ScopedRef] for consistency (like the title says), but the proposal says [Scoped] 😄

[cston]

Thanks @Sergio0694. Updated.