[WIP] Use latest Omnisharp-rosyln for .NET 6 support

[mispencer]

The version of Omnisharp-rosyln doesn't support .NET 6. To provide this support, we upgrade to the latest Omnisharp-roslyn, and use the .NET 6 build of it.

Issues not necessarily resolvable:

• This requires the users to have .NET 6 SDK installed on this machines
• There appears to be no build of Linux x86 to use

Current issues to be resolved:

• [ ] The windows x86 tests are failing
• [ ] This doesn't appear to be compatible with .NET framework in non-Windows OSs
• [x] This is a beta release of Omnisharp-roslyn

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This change is 

[puremourning]

Thanks! Sorry for the delay - I've been on vacation and had covid.

Worth me looking at it now, or wait for the issues to be resolved?

[RA-Kooi]

This doesn't appear to be compatible with .NET framework in non-Windows OSs

I can confirm the .NET 6.0 (non-mono) version works for me on Arch Linux and Gentoo Linux.

This is a beta release of Omnisharp-roslyn

This is a while ago, I think we can assume it's in stable now?

[mispencer]

That is correct. There is a stable release now.

[bstaletic]

@mispencer I have rebased this branch locally and pushed to my fork. However, I can't figure out what omnisharp is doing, as diagnostics and completion all seem pretty off from what ycmd expects in its tests.

Here's a link https://github.com/bstaletic/ycmd/tree/OmnisharpLatest