Remove hardcoding of .NET version from the .NET 6 publish

[333fred]

Currently we're including -net6.0 in our publish urls for OmniSharp. This is bad because we cannot programmatically determine, from a client, whether .NET 6 is the current version of OmniSharp, and we have to hardcode the net6.0 part. This will then require a rev of the extension every time we want to update the .NET version, which is brittle. We should do one of two things:

1. Update the URL to not include a version number. Something like just -net, for example, though that does seem a bit odd (if this is the .NET version, what's the other one exactly?).
2. Update the latest version info we publish to include the version of .NET it uses, and also include that info in the blob storage we publish for an individual release, so that clients can programmatically ask for that info, then construct the correct download URLs.