refactor: replace boost::filesystem with std::filesystem

Warning: Replacing fs::system_complete calls with fs::absolute calls
in this commit may cause minor changes in behaviour because fs::absolute
no longer strips trailing slashes; however these changes are believed to
be safe.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>
Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

src/net.cpp

@@ -16,6 +16,7 @@
 #include <compat.h>
 #include <consensus/consensus.h>
 #include <crypto/sha256.h>
+#include <fs.h>
 #include <i2p.h>
 #include <net_permissions.h>
 #include <netaddress.h>