Rename internal Utf8String to MdUtf8String (#17856)

Rename internal Utf8String to MdUtf8String (#17856)

We want to start prototyping Utf8String in CoreFxLab
and for that, we'll need a bare-bones System.Utf8String
class exposed from System.Private.CoreLib.

Unfortunately, CoreLib already has an internal
struct named System.Utf8String. Since it's only
an internal type, we'll exercise eminent domain
on its name now and get these noise changes out of
the way.