NEXUS-6099: Desupport Nx 1.x version plugin configurations

Removing upgrades happening in Nexus 1.x lifecycle, allowing configuration upgrades only from model versions that were released since Nexus 2.0.

Core got new model version 2.8.0 with no actual upgrade steps yet (merely re-versioned existing 2.7.0 model). This can change later.

Also, some plugin configurations are "trivial" (Kenai is read only), removing all the unneeded cruft from them.

Issue (part 2, plugins):
https://issues.sonatype.org/browse/NEXUS-6099

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF8