Upgrade to jdk 21

[adejanovski]

Project board link

We'd need to upgrade to the latest LTS JDK and update our dependencies accordingly. In the process, we should also consider switching to dropwizard 4.0 and dropwizard-cassandra, since we've been relying so far on a 3rd party bundle which is no longer supported.

This may also be the right time to ditch Shiro as authentication system and use dropwizard built in authc/authz features.

Definition of Done

• [ ] Reaper runs on JDK 17 or 21
• [ ] Reaper uses Dropwizard 4.0.x
• [ ] Reaper uses the official dropwizard-cassandra bundle
• [ ] Shiro is no longer used for authentication and replaced by the built in dropwizard auth features