Add net6.0 targets

[MihaZupan]

Blocked on dotnet/aspnetcore#32364 as some of the health check tests are hitting this.

Fixes #936

[MihaZupan]

Bumped the SDK to 6.0.100-preview.5.21263.3 (lastest build off the installer repo).

Everything is now buildable and testable in 6.0, with the exception of the Auth Sample. That one is hitting the following error:

  PDB0021: Document name doesn't match any pattern in Source Link: 'd:\workspace\_work\1\s\samples\ReverseProxy.Auth.Sample\Views\Account\AccessDenied.cshtml'
  PDB0021: Document name doesn't match any pattern in Source Link: 'd:\workspace\_work\1\s\samples\ReverseProxy.Auth.Sample\Views\Account\LoggedOut.cshtml'
  PDB0021: Document name doesn't match any pattern in Source Link: 'd:\workspace\_work\1\s\samples\ReverseProxy.Auth.Sample\Views\Account\Login.cshtml'
D:\workspace\_work\1\s\.packages\microsoft.dotnet.arcade.sdk\6.0.0-beta.21263.1\tools\SymStore.targets(70,5): error MSB3073: The command ""D:\workspace\_work\1\s\.packages\microsoft.diasymreader.pdb2pdb\1.1.0-beta2-19575-01\tools\Pdb2Pdb.exe" "D:\workspace\_work\1\s\artifacts\bin\ReverseProxy.Auth.Sample\Release\net6.0\ReverseProxy.Auth.Sample.dll" /out "D:\workspace\_work\1\s\artifacts\SymStore\Release\ReverseProxy.Auth.Sample\net6.0\ReverseProxy.Auth.Sample.pdb" /srcsvrvar SRC_INDEX=public" exited with code -1. [D:\workspace\_work\1\s\samples\ReverseProxy.Auth.Sample\ReverseProxy.Auth.Sample.csproj]

[MihaZupan]

While CI is happy with this, this should further wait for dotnet/sdk#17461 to be fixed so that we can continue working locally.

It was useful that we did this early to spot issues like the AspNetCore bug but it shouldn't be criticial to merge ASAP.

[MihaZupan]

@Tratcher this PR is now unblocked with the latest SDK+VS version.

I had to revert this #if until we start consuming future 6.0 builds with the json fix.