Modernize lock object declarations from object to System.Threading.Lock

Replace all lock object declarations using 'object' type with the C# 13
System.Threading.Lock type across 12 files. The Lock type provides a
more efficient locking mechanism - when used with lock(), the compiler
automatically uses Lock.EnterScope() instead of Monitor.Enter/Exit.

Also made two previously non-readonly lock fields readonly:
- McClient.DigLock
- Protocol18.MessageSigningLock

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

MinecraftClient/Protocol/Handlers/Protocol18.cs

@@ -102,7 +102,7 @@ namespace MinecraftClient.Protocol.Handlers
         private int oldSamplesWeight = 1;
 
         private bool receiveDeclareCommands = false, receivePlayerInfo = false;
-        private object MessageSigningLock = new();
+        private readonly Lock MessageSigningLock = new();
         private Guid chatUuid = Guid.NewGuid();
         private int pendingAcknowledgments = 0, messageIndex = 0;
         private LastSeenMessagesCollector lastSeenMessagesCollector;