Switched to the more performant System.Threading.Lock on .NET 9.0+

[MarkCiliaVincenti]

Please check the following before creating a Pull Request

• If this is a new feature or piece of functionality, have you started a discussion and gotten agreement on it?
• If it fixes a bug or problem, is there an issue to track it? If not, create one first and link it please so there's clear visibility.
• Did you write tests to ensure you code works properly?