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>
