Update to the latest SDKs and Package versions

[JoeRobich]

• Add validation against the .NET 8 preview 4 SDK
• Move to latest release .NET 7 SDK version
• Move to the latest .NET 6 SDK version still support by Mono MSBuild 6.0.203
• Update Roslyn to 4.7.0-2.23225.15
• Move test projects from netcoreapp3.1 to net6.0 and from netstandard 1.3 to netstandard2.0

[JoeRobich]

Ugh, Mono ships with a MSBuild that incompatible with the newer SDKs. Will have to revisit this later. Perhaps stay on the older 6.0.2xx SDK for test projects and skip the net7/net8 tests on Mono.

[JoeRobich]

Downgraded .NET 6 SDK back to a 6.0.2xx release. Long term we should keep separate projects for testing OmniSharp.MSBuild against Mono and SDK. Not sure we are exercising framework style project loading under Mono at all.