Update example service to use up-to-date JDK and dependencies #109
Description
The service uses Java 11, which is now out of support. We should update it to use the current LTS version.
Similarly, the Micronaut version is several major versions old, and should be updated to a supported version - we should not teach people that it is OK to use unsupported software.
While we're at it, we should check the remaining dependencies and update them to recent versions as well.