refactor: Rename LockGuard to StdLockGuard for consistency with StdMutex

Summary:
This is part [2/5] of Core PR18635 : https://github.com/bitcoin/bitcoin/pull/18635/commits/dfb75ae49d4d617ec02188a6f449e8b8015ad467

Depends on D7877

Test Plan:

ninja all check-all

Check that clang's thread safety analysis is happy.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7878