Update dependencies and cleanup projects

[0xced]

• Update test projects from .NET Core 3.1 (out of support) to .NET 6 (LTS)
• Remove <Service Include="{82a7f48d-3b50-4b1e-b82e-3ada8210c358}" /> (see Test projects end up with this GUID microsoft/vstest#472 (comment))
• Remove <LangVersion>7.2</LangVersion> on test projects
• Fix Run_Specifies_Cancellation_Token which (rightfully) throws when running on .NET 6

[kfcampbell]

I'm interested in picking this up, as I'm looking at #312 but also want to update other aspects of the library. To avoid stomping on your branch/fork any further, I'll create a separate PR.

[0x5bfa]

@Lamparter i think we've already worked this in our PR right?

[Lamparter]

@Lamparter i think we've already worked this in our PR right?

Yes I think so. I definitely did do 1st (obviously) and I remember fixing the 4th task also. But not sure whether or not I implemented 2nd and 3rd.

• Update test projects from .NET Core 3.1 (out of support) to .NET 6 (LTS)
• Remove <Service Include="{82a7f48d-3b50-4b1e-b82e-3ada8210c358}" /> (see Test projects end up with this GUID microsoft/vstest#472 (comment))
• Remove <LangVersion>7.2</LangVersion> on test projects
• Fix Run_Specifies_Cancellation_Token which (rightfully) throws when running on .NET 6

[0x5bfa]

#321 supersedes this PR, @Lamparter we seem to haven't implemented the 4th and sounds like we should separate.

[Lamparter]

Separate?

[0x5bfa]

Instead of including that change in #321

[Lamparter]

Where would we include that change instead?

[0x5bfa]

A new PR or here (with other changes reverted as superseded by our PR).

[Lamparter]

This isn't our pr though. I think it's best to just leave in one pr.