Replace Bintray/JCenter

[keilw]

JFrog just announced it will shut down Bintray and JCenter by the end of April: https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

This affects multiple aspects of the UoM build process or download content:

• Downloads - Replace a few parts of https://bintray.com/unitsofmeasurement/downloads: We should move the JIRA archives, JSR363MailingLists and JSR363MeetingMinutes to either Google Drive or Dropbox. All the specifications only stored as a convenience are available on JCP.org anyway and can be discarded with Bintray.
• Maven - Change additional repositories in every relevant POM. While "Artifactory installations with remote repositories configured against Bintray and Centers will not be affected by this sunset." is worth clarification, it does not sound like this applies to anything other than an Artifactory client but not e.g. a build job on a Circle-CI, the snapshot repositories like https://oss.jfrog.org/artifactory/oss-snapshot-local are most likely going away at least those few weeks after the sundown
• Maven - Deployment to Bintray Snapshot repositories won't work anymore. Trying that after May 1st will certainly break the build, so any use of CI_DEPLOY_USERNAME/PASSWORD or equivalent must point to something else, probably a Sonatype/Nexus user with appropriate permission to perform snapshot deployments
• P2 - Eclipse P2 Update Sites under https://bintray.com/unitsofmeasurement/p2 are only used by the Tycho build of Eclipse UOMo. For anything that is not made available via Eclipse Orbit anyway, there should be an alternate location e.g. via https://www.eclipse.org/uomo/download.php
• RPM - Repositories on https://bintray.com/unitsofmeasurement/rpm These should be taken care of by the PCP maintainers, otherwise they will be gone after the sundown, see the groups.io thread.