Debian and RPM packages: mark Erlang 27 as supported

[michaelklishin]

Since very few people watch https://github.com/rabbitmq/rabbitmq-packaging, I'm filing this for visibility of rabbitmq/rabbitmq-packaging#55, which is ready to go with 4.0.4 or a later version.

4.0.3 had a number of fairly important bug fixes, so the Erlang 27 "promotion" had to wait.

References #12208 and a number of erlang/otp issues we probably should not list to avoid link spam.