Stop building on Gerrit v2.10/v2.11/v2.12

Do not build anymore Gerrit and its plugins on v2.10/v2.11/v2.12
which are mostly obsolete and not supported anymore by the community.

This allows to save disk space on Gerrit-CI and get rid completely
of Java 7.

Change-Id: I3359120941cd98dc7944813fce35c1149eac481a