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/Settings.cs

@@ -1011,7 +1011,7 @@ namespace MinecraftClient
                 private readonly Dictionary<string, object> VarObject = new();
 
                 [NonSerialized]
-                readonly object varLock = new();
+                readonly Lock varLock = new();
 
                 /// <summary>
                 /// Set a custom %variable% which will be available through expandVars()