Sign with GPG
Recently, the old-style deployment to the Maven central repository was deprecated, and now we need to use Sonatype OSS repository to put project artifacts to sync with the Maven central.
Sonatype repository requires us to sign every jar with GPG. https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide
org.xerial group ID has already been granted to deploy the artifacts to the Sonatype repository. The remaining task is to change pom.xml so that it uses Sonatype repo and gpg-plugin.