Puppet Version: 5 or 6
Puppet Server Version: 5.x or 6.x
OS Name/Version: N/A
Latest Puppet Server 5.x or 6.x release uses JRuby 9.2, which is Ruby 2.5. This also matches the Ruby that ships with Puppet and will be a supported version for quite a while.
Puppet Server 5.x or 6.x uses JRuby 9.1, which is Ruby 2.3. Ruby 2.3 is EOL in March of 2019.
I presume there are already plans for a minor point release around that time to bump the jruby version but opening this just in case