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.

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
TBD