Use IHttpClientFactory Instead of Directly Constructing HttpClient

[halforbit]

This article outlines the motivations for the move:
https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests

Unfortunately, this is only supported in .NET Core 2.1+, so to retain backwards compatability, this will need to be switched off of target framework, with legacy .NET Framework 4.x continuing to use the existing pattern.