Fix seedstack build/test
The job that is currently running the test is failing: https://gitlab.ow2.org/ow2/oscar/seedstack/-/jobs/35613
After a very quick look at the logs I realized that using maven:latest
Docker image (cf https://gitlab.ow2.org/ow2/oscar/seedstack/-/blob/master/.gitlab-ci.yml#L8) is leading to the usage of Java 15 whereas SeedStack most likely requires Java 8 or 11.
Anyway this will require further investigation.
/cc @mh