NEXUS-5828: Upgrade step for security manager

Changes:
* upping security config version from 2.0.4 to 2.0.5
* added upgrade step, that sets security manager to "default"
* cleanup of Upgrade203to204

Issue
https://issues.sonatype.org/browse/NEXUS-5828

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF26