RabbitMQ 3.12.0 is now GA

[michaelklishin]

Hi @tianon @yosifkit 👋🙂

RabbitMQ 3.12.0 has gone GA and was officially declared as compatible with Erlang 26.

It has some non-trivial improvements (one, two), so many people will be eager to get it.
I don't know how exactly the process works but we should convert the 3.12 flavor of this image
to use GA releases :)

On top of that, since we (Team RabbitMQ) declared it as Erlang 26-compatible from the start,
we can resolve #636 by updating the 3.12 version to use Erlang 26.

With over one month of daily Erlang 26 use, many load tests and so on, we haven't found a single
easily observable issue. So, let's optimistically assume that version is as good as 25.3.x in
terms of stability. On many workloads it provides a 5-10% throughput gain for free.

[michaelklishin]

Someone has reminded me that those upgrading RabbitMQ and Erlang to 26 at the same time must be on RabbitMQ 3.11.17 or later.

Let's hold off the move to Erlang 26 for a few months to make sure that more environments
are upgraded to 3.11.17+.