versions.md: Add release calendar for RabbitMQ 3.9.0
versions.md: Add release calendar for RabbitMQ 3.9.0

It takes into account a single release candidate then the final release.
However, it has room for additional release candidates if needed. This
calendar is meant to be updated accordingly during the release cycle.

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website