Use sbt-pgp's publishSigned task in sbt-release release process

This PR replaces the `publishArtifacts` release step of sbt-release's release process by a custom release step which uses sbt-pgp's publishSigned task.

Note : I couldn't properly test this change, since this means that I'd need to make a release.
However, this mimics as closely as possible the `publishArtifacts` release step implementation.