Use HttpClientHandler for bounded WebRequestHandler creation

Replace a resolved WebRequestHandler constructed directly as the handler argument of HttpClient, or returned directly from a method declared to return HttpClientHandler, with HttpClientHandler. Only an empty initializer or assignments to shared HTTP settings are supported. Handler variables, subclasses, certificate callbacks, caching, and Framework-specific timeouts require manual migration. Framework process-wide ServicePointManager and configuration settings are not transferred to the modern HTTP stack.