deps: upgrade CppWinRT to 2.0.200316.3, gsl to v2.1.0

[DHowett-MSFT]

This commit upgrades C++/WinRT to 2.0.200316.3 and fixes a couple of the
transitive dependency issues we had in the process.

Because the latest version has better dependency resolution, we're able
to properly depend on Microsoft.UI.Xaml and the Toolkit in TerminalApp
and TerminalAppLib so we no longer need to manually include .dll and
.pri files.

Because of nebulous other changes in dependency resolution,
WindowsTerminalUniversal isn't picking up transitive .winmd dependencies
from TerminalApp, and needs to include them as ProjectReferences
directly. This was already happening transitively, so now it's explicit.

I've also taken the time to upgrade GSL to v2.1.0, the last release
before they removed span::at and blew up our world.

[zadjii-msft]

You know, I viewed this back when it was first pushed and I don't think I had any comments, but never hit the "Review Changes" button. I'm doing this now to get it out of my queue.

I dunno if we need this to land before 1.0, but so far looks fine to me

[DHowett-MSFT]

Completely anecdotal: I've been having issues with incremental rebuild across launches of Visual Studio -- like, pressing F5 on a new launch would fail because of winmd or xbf files. I just tried that with this change and it seems like it's better? Somewhat better.

[zadjii-msft]

like, pressing F5 on a new launch would fail because of winmd or xbf files

I see that all the time, especially when switching branches. There doesn't seem to be any rhyme or reason, but I'd be happy if this helped even a little.

[zadjii-msft]

K cool with me. Kinda shocked there wasn't more that needed to be done, but I guess that's a good thing

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