Package updates

[lukebakken]

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[lukebakken]

@bording thanks! I tried upgrading System.Memory via Visual Studio and it wanted to bring in other packages (I forget which) but that doesn't seem to be the case here.

[bording]

@lukebakken I was confused for a second when I didn't get a prompt to create a new PR, but I see that you have already! 😄

I pushed the branch up and then noticed a problem. You know how I was saying it could be risky to bring in packages like System.Memory when using "old" .NET Framework versions? Turns out we were hitting a problem and the new package revealed it.

The older System.Memory package was causing the net461 target to be referencing the wrong System.Net.Sockets assembly, It was referencing a newer version that isn't actually part of .NET Framework 4.6.1 but will be on the system if a newer version of .NET Framework is installed.

That was giving the code access to a ConnectAsync overload that it shouldn't have had available, so I had to reintroduce the #ifdef around it.

[lukebakken]

@bording you just answered the question I was about to ask ... which was "how in the heck did these changes expose that build failure???"

At least the AppVeyor build revealed that error.