Drop puppetca_puppet_cert provider
Currently for Puppet 6.0 or higher the Puppetserver REST API is used. Puppet 5 is EOL so this can be dropped. Older versions (dating back to Puppet 4) had an incomplete API, but core methods do work. Just the reporting in the UI is incomplete. This means even with Puppet 5 it won't be completely broken.
Dropping this allows us to move forward. The current implementation has some coding techniques which need adjustment on Ruby 3. Since nobody should be using the old code, refactoring it is pointless.