Migrate to .NET 9 (#198)

src/Ryujinx.Memory/SparseMemoryBlock.cs

@@ -14,7 +14,7 @@ namespace Ryujinx.Memory
 
         private readonly PageInitDelegate _pageInit;
 
-        private readonly object _lock = new object();
+        private readonly Lock _lock = new();
         private readonly ulong _pageSize;
         private readonly MemoryBlock _reservedBlock;
         private readonly List<MemoryBlock> _mappedBlocks;