fix compilation warnings

Author: adamsitnik

src/System.CommandLine/Invocation/ProcessTerminationHandler.cs

@@ -13,7 +13,9 @@ internal sealed class ProcessTerminationHandler : IDisposable
     private readonly CancellationTokenSource _handlerCancellationTokenSource;
     private readonly Task<int> _startedHandler;
     private readonly TimeSpan _processTerminationTimeout;
+#if NET7_0_OR_GREATER
     private readonly IDisposable? _sigIntRegistration, _sigTermRegistration;
+#endif
 
     internal ProcessTerminationHandler(
         CancellationTokenSource handlerCancellationTokenSource, 
@@ -25,7 +27,7 @@ internal ProcessTerminationHandler(
         _startedHandler = startedHandler;
         _processTerminationTimeout = processTerminationTimeout;
 
-#if NET7_0_OR_GREATER
+#if NET7_0_OR_GREATER // we prefer the new API as they allow for cancelling SIGTERM
         if (!OperatingSystem.IsAndroid() 
             && !OperatingSystem.IsIOS() 
             && !OperatingSystem.IsTvOS()
@@ -43,16 +45,17 @@ internal ProcessTerminationHandler(
 
     public void Dispose()
     {
+#if NET7_0_OR_GREATER
         if (_sigIntRegistration is not null)
         {
             _sigIntRegistration.Dispose();
             _sigTermRegistration!.Dispose();
+            return;
         }
-        else
-        {
-            Console.CancelKeyPress -= OnCancelKeyPress;
-            AppDomain.CurrentDomain.ProcessExit -= OnProcessExit;    
-        }
+#endif
+
+        Console.CancelKeyPress -= OnCancelKeyPress;
+        AppDomain.CurrentDomain.ProcessExit -= OnProcessExit;    
     }
 
 #if NET7_0_OR_GREATER