Hi @aragot,
I believe the Maven packaging names (milestones) for these EAP releases follow a different numbering scheme than the EAPs. Earlier release posts made this clearer.
The current EAP06 corresponds to m0015, so your jira.version variable should read 8.0.0-m0015. The mapping is detailed at the top of the page you linked.
As for the artifactory, I have personally found the directory listing at Index of maven-external/ easier to use. The jira-api package for example, can be found at Index of maven-external/com/atlassian/jira/jira-api/8.0.0-m0015
Cheers,
Tobi