The NativeCallableAttribute was recently made public. During subsequent C# function pointer discussions it was mentioned the naming of the attribute is inconsistent with the C# function pointer proposal. It has been decided to rename the attribute to UnmanagedCallersOnlyAttribute.

The intent here is to align with the C# function pointers usage of the unmanaged contextual identifier.

The following should be performed:

• Update official API and ref assembly.
• Update test usage.
• Update various places in runtime source to align with rename.

/cc @jkoritzinsky @elinor-fung