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 https://github.com/Microsoft/vstest/issues/472#issuecomment-378811616)
• Remove <LangVersion>7.2</LangVersion> on test projects
• Fix Run_Specifies_Cancellation_Token which (rightfully) throws when running on .NET 6

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[kfcampbell]

https://github.com/octokit/octokit.graphql.net/pull/313/commits/4661572495ec86fd03b540e800cc727a93157b26 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.