Publish JARs to Maven Central

[msgilligan]

For the early releases we have been publishing JARs to the GitLab Maven Repo. We will continue to do this for 0.2 and any 0.2.x releases.

We will publish v0.3 to Maven Central.

[msgilligan]

Note: that project-level GitLab endpoint URLs now require authentication. For anonymous access, the group URL endpoints must be used. See: https://docs.gitlab.com/user/packages/maven_repository/

The Group Endpoint URL for bitcoinj on GitLab is https://gitlab.com/api/v4/groups/2964446/-/packages/maven.