Replace boost::filesystem with std::filesystem

include/caosdb/certificate_provider.h

include/caosdb/configuration.h

include/caosdb/connection.h

include/caosdb/entity.h

include/caosdb/file_transmission/file_reader.h

include/caosdb/file_transmission/file_writer.h

include/caosdb/utility.h

src/caosdb/configuration.cpp

src/caosdb/file_transmission/download_request_handler.cpp

src/caosdb/file_transmission/file_reader.cpp

src/caosdb/file_transmission/file_writer.cpp

src/caosdb/file_transmission/upload_request_handler.cpp

src/caosdb/transaction.cpp

src/ccaosdb.cpp

test/test_file_transmission.cpp