Nexus search collector plugin supported purl types fix

Checklist

• documentation is changed or added in ./docs
• unit tests updated to test changes

Description

• Check supported types of plugin instance rather than class
• Set default value of CollectNexusSearch.supported_purl_types to all purl types except git, github, and gitlab
  • Remove unneeded override of supports_purl_type() method as a result
• Update base image for CI and release Dockerfiles
  • Previous base image was failing during apt-get update due to a broken image or changes to the Ubuntu archive source

Closes #285 (closed)