feat: breaking change: revive cleanup projects PR, target .NET 6.0

[kfcampbell]

BREAKING CHANGE: target .NET 6.0.

Builds the project on .NET 8. Also makes the following changes of @0xced's from #304:

• 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

[github-actions]

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[kfcampbell]

4661572 has a test "fix" for a serialization issue that occurs when updating .NET versions. Essentially, the expected string query string in tests looks like (note the x => new piece):

data => Rewritten.Value.Select(
    data["data"]["repository"]["issue"],
    x => new
    {
        Value = Rewritten.Value.SingleOrDefault(
            Rewritten.Value.Select(
                x["value"],
                y => new
                {
                    Closed = y["closed"].ToObject<bool>(),
                    Description = y["description"].ToObject<string>()
                }))
    })

and the actual string looks like (note the x => new object piece):

data => Rewritten.Value.Select(
    data["data"]["repository"]["issue"],
    x => new object
    {
        Value = Rewritten.Value.SingleOrDefault(
            Rewritten.Value.Select(
                x["value"],
                y => new object
                {
                    Closed = y["closed"].ToObject<bool>(),
                    Description = y["description"].ToObject<string>()
                }))
    })

These are the "same" content, but fail string comparisons. I'm not sure how to monkey with our string comparison library or our query generation to get these to be truly identical. @StanleyGoldman, since you originally introduced this comparison mechanism years ago, I'd be very interested to know if you have thoughts!

In the meantime, my hacky "fix" is a find/replace that looks for instances of "new{" in the expected string and replaces them with "newobject{" to match the actual strings. I don't love this approach, hence I'm leaving this PR in draft for the time being.

@nickfloyd and @0xced, I'm also interested to hear if you have comments on this.

[nickfloyd]

Yeah this feels like something is off with serialization. I'm not sure what the proper call would be here. Let me pull this down today and see if I can get a better picture of what might be going on.