우분투 터미널에서 sudo apt update 명령을 사용했더니, 아래의 오류 메시지가 나온다. 더보기 W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://packages.ros.org/ros/ubuntu bionic InRelease: The following signatures were invalid: EXPKEYSIG F42ED6FBAB17C654 Open Robotics W: Failed to fetch http://packages.ros.org/ros/ubuntu/dists/bionic/..
