Migrate to .NET 9 (#198)

src/Ryujinx.Input.SDL2/SDL2Keyboard.cs

@@ -4,6 +4,7 @@ using System;
 using System.Collections.Generic;
 using System.Numerics;
 using System.Runtime.CompilerServices;
+using System.Threading;
 using static SDL2.SDL;
 
 using ConfigKey = Ryujinx.Common.Configuration.Hid.Key;
@@ -17,7 +18,7 @@ namespace Ryujinx.Input.SDL2
             public bool IsValid => To is not GamepadButtonInputId.Unbound && From is not Key.Unbound;
         }
 
-        private readonly object _userMappingLock = new();
+        private readonly Lock _userMappingLock = new();
 
 #pragma warning disable IDE0052 // Remove unread private member
         private readonly SDL2KeyboardDriver _driver;