Remove modules for perl 5.16, 5.18 (and 5.20?)

[mojca (Mojca Miklavec)]

Here's a list of things we have to do (please add the missing items):

• empty the p5-graveyard port; we no longer need replacements for 5.8-5.14
• decide whether to keep or remove support for 5.20 (if we wait long enough with the ticket, 5.24 will be out anyway, so we may just as well remove 5.20 now :); on the other hand that should not mean removing all perl5 variants just because there will be a single perl left until introduction of 5.24
• close #48365
• potentially collect a list of affected ports and warn maintainers (maybe try to make sure that we don't add a huge number of CC-ed users that will want to unsubscribe straight away while we still discuss what to do)
• handle subversion-perlbindings
• remove 5.16-5.x from perl5.branches from ports with variants
• remove 5.16-5.x from all p5-foo modules
• fill in the p5-graveyard port
• (potential removal of perl5.x itself can wait)