Switch RabbitMQ generic-unix URL to Bintray
Switch RabbitMQ generic-unix URL to Bintray

Just noticed that
https://ci.rabbitmq.com/teams/main/pipelines/server-release:v3.7.x/jobs/create-rc-github-release/builds/30
stopped publishing RC artefacts since 3.7.9-rc.3. Until that gets
addressed, switching to Bintray, our other distribution channel.

docker-library/rabbitmq,infosiftr/rabbitmq