refactor: Do not discard try_lock() return value

Summary:

Microsoft's C++ Standard Library uses the [[nodiscard]] attribute for try_lock().
See: https://github.com/microsoft/STL/blob/main/stl/inc/mutex

It appears the same issue occurs with GCC 13.1.

Backport of core#26189.

Test Plan:
With debug and GCC 13.1:

ninja all check-all

It should return no warning.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14005