This is a partial backport of core#22937 [2b/2g]
https://github.com/bitcoin/bitcoin/pull/22937/commits/6544ea5035268025207d2402db2f7d90fde947a6
The backport for this commit was split in to multiple commits to make review easier. This part deals with converting fs::string() calls that
will become unsafe when migrating from boost::filesystem to std::filesystem.
Depends on D10795