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.