microsoft/botbuilder-dotnet#4860

Unfortunately we broke developers who implemented their own ITurnContext when adding a cast to TurnContext in DialogContext.GetLocale(). Furthermore, the GetLocale() method is quite useless considering that whenever the DialogContext is available, a ITurnContext instance is available as well and we could get the locale from there.

I have some broader locale changes in another incoming PR (in which I mark DialogContext.GetLocale() as deprecated among other things), but this is the minimum bar, which is to un-break developers that have been broken because of this change.