Add 3.x SignalR background service sample (#17318)

* Add 3.0 SignalR background service sample

* Fix background-services.md date

* Compressed a few monikers

* Renamed 3.0 sample to 3.x

* Apply suggestion

Co-Authored-By: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>

* Shrink ClockHubClient.StopAsync snippet by 1 line

Also tweaked the wording slightly in the "any class that inherits from hub..." note

* Remove future tense from note

* Use async On overload in 3.x sample

Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>

Author: UncleDave

aspnetcore/signalr/background-service/sample/Clients.ConsoleOne/Clients.ConsoleOne.csproj renamed to aspnetcore/signalr/background-service/samples/2.2/Clients.ConsoleOne/Clients.ConsoleOne.csproj

@@ -58,6 +58,6 @@ public Task StopAsync(CancellationToken cancellationToken)
         {
             return _connection.DisposeAsync();
         }
-    }
 #endregion
+    }
 }

aspnetcore/signalr/background-service/sample/Clients.ConsoleOne/Program.cs renamed to aspnetcore/signalr/background-service/samples/2.2/Clients.ConsoleOne/Program.cs

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>