Fix the deprecation warnings from new boost versions

Summary: A better solution would be to remove boost fs and boost system depedencies, but since it's a huge work (see https://github.com/bitcoin/bitcoin/pull/20744) let's fix the annoying warnings for now.

Test Plan:

ninja all check

Make sure there is no warning anymore

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13175