Use HttpClient instead of WebClient for the updater

[PiKeyAr]

Since the .NET 5.0 upgrade it's impossible to actually cancel an update for SA Tools in the Hub. This is because the implementation of WebClient is different between Framework and Core. WebClient in .NET 5.0 and higher no longer cancels the download in progress.

Microsoft treat WebClient as a deprecated class so they don't want to fix it. HttpClient is the recommended replacement but it lacks some features WebClient has out of the box, so it needs some workarounds to display download progress.

This issue will also affect the Mod Loader rewrite if it is upgraded to 5.0 or later.

[PiKeyAr]

Useful comment: dotnet/runtime#31479 (comment)

[MainMemory]

Maybe we just find some library that does what we want, or make something ourselves using raw streams.