Merge #13982: refactor: use fs:: over boost::filesystem::

Summary:
4b3b85c597 refactor: use fs:: over boost::filesystem:: (fanquake)

Pull request description:

Noticed while investigating #13973.

Tree-SHA512: 20a764d2ff460883fa0fd543c0a51031a9a202b40cfda9943f9995d3108c0a8296a3982b63bbd069167f73a1855003304a83df466763032ce7339b08b3a97d4b

Backport remaining of Core PR13982
https://github.com/bitcoin/bitcoin/pull/13982/

Review note: changes to qt/guiutil.cpp were made in D3219. Changes to
wallet/rpcdump.cpp were made in D1165.

Test Plan:

make check
./bitcoind

Verify that debug.log is being written to.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D5057