Remove suppression and fix VSTHRD100 (#78373)

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2458776

Author: sharwell

src/LanguageServer/Microsoft.CommonLanguageServerProtocol.Framework/AbstractLanguageServer.cs

@@ -12,6 +12,7 @@
 using System.Reflection;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.VisualStudio.Threading;
 using StreamJsonRpc;
 
 namespace Microsoft.CommonLanguageServerProtocol.Framework;
@@ -326,18 +327,21 @@ private ValueTask ShutdownRequestExecutionQueueAsync()
         return queue.DisposeAsync();
     }
 
-#pragma warning disable VSTHRD100
     /// <summary>
     /// Cleanup the server if we encounter a json rpc disconnect so that we can be restarted later.
     /// </summary>
-    private async void JsonRpc_Disconnected(object? sender, JsonRpcDisconnectedEventArgs e)
+    private void JsonRpc_Disconnected(object? sender, JsonRpcDisconnectedEventArgs e)
     {
-        // It is possible this gets called during normal shutdown and exit.
-        // ShutdownAsync and ExitAsync will no-op if shutdown was already triggered by something else.
-        await ShutdownAsync(message: "Shutdown triggered by JsonRpc disconnect").ConfigureAwait(false);
-        await ExitAsync().ConfigureAwait(false);
+        JsonRpc_DisconnectedAsync(sender, e).Forget();
+
+        async Task JsonRpc_DisconnectedAsync(object? sender, JsonRpcDisconnectedEventArgs e)
+        {
+            // It is possible this gets called during normal shutdown and exit.
+            // ShutdownAsync and ExitAsync will no-op if shutdown was already triggered by something else.
+            await ShutdownAsync(message: "Shutdown triggered by JsonRpc disconnect").ConfigureAwait(false);
+            await ExitAsync().ConfigureAwait(false);
+        }
     }
-#pragma warning disable VSTHRD100
 
     internal TestAccessor GetTestAccessor()
     {