Merge vNext into master

[333fred]

No description provided.

[filipw]

This is the summary of what this change brings:

1. There is now a new .NET 6.0 build of OmniSharp published as build artifact. It gets published for the following RIDs

• win7-x64
• win7-x86
• win10-arm64
• osx-x64
• osx-arm64
• linux-x64
• linux-arm64 (added later after this PR in Add Linux arm64 net6 build. Only build Windows net6 packages on Windows. #2293)

The previous .NET Framework targets for Windows and Mono based variants for MacOS and Linux (including arm64) are still all there and OmniSharp is cross compiled for them.

2. The .NET 6.0 build uses only .NET SDK MSBuild on *nix. On Windows it queries for VS instances as long as they come with SDK 6. In either case .NET SDK 6.0.100 must be installed and no newer SDK must be available. It has to be official install - source built SDK is not supported due to NuGet versioning differences.
3. When new .NET 6 SDK comes out, OmniSharp will not be able to handle it until it is upgraded to that version (this is caused by having runtime dependency on NuGet assemblies, which are typically revved up in SDK releases)
4. Both HTTP and STDIO servers have been upgraded to .NET 6.0.
5. The 6.0 build is almost a drop-in replacement for the old server. It is compatible functionally but cannot be executed on its own, because it is not built as self-contained application - we require dotnet SDK to be installed anyway, so there is no point in building like that. Because of that, .NET 6 build of OmniSharp is started using dotnet OmniSharp.dll. To achieve this with VS Code you need to have at least v1.23.18-beta2 installed, with the following settings:

{
    "omnisharp.useModernNet": true,
    "omnisharp.path": "latest"
}

6. Non-SDK-style .NET Framework projects are not supported by the .NET 6.0 build. For opening legacy .NET Framework projects, the global Mono O# variant is still needed (*nix) or the .NET Framework O# variant (Windows), as in the past.
7. Ready to Run is enabled Improve startup performance by enabling ReadyToRun. #2303

[filipw]