(maint) Bump to puppet6
(maint) Bump to puppet6

This commit changes the final and nonfinal repo targets to the puppet6 stream.
Previously, we had only done that in build_defaults, but it's also needed here.

puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppetserver