net_processing: add thread safety annotation for m_highest_fast_announce

Summary:
This concludes backport of core#26036
https://github.com/bitcoin/bitcoin/pull/26036/commits/d575a675cc884b1bebdb6197f2ef45c51788d4a3

Depends on D15286

Test Plan:
With Debug:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15287