net/rabbitmq: update to v3.12.13 and new tarball

[dch]

Summary

RabbitMQ has long provided a generic unix tarball which is better
suited for a FreeBSD port. There should be no functional changes
for this approach, and it simplifies build and release by aligning
better with upstream deployment, removing dependency on lang/elixir
releases.

https://rabbitmq.com/install-generic-unix.html