Upgrade to mantic: RabbitMQ upgrade 3.10.8 -> 3.12 fails

[Electronic]

In Ubuntu Lunar, the rabbitmq-server version is 3.10.8, in Ubuntu Mantic, it is 3.12.1. RabbitMQ 3.12 requires a feature flag `classic_queue_type_delivery_support`, which was only added in 3.10.9. A feature flag needs to be enabled before the upgrade, but it cannot be enabled from the Lunar version.
(Feature flag `classic_mirrored_queue_version` is also required by 3.12 and is not automatically enabled, but at least downgrading and enabling it works).

Therefore, the upgrade path for rabbitmq-server using only Ubuntu-provided packages is broken; I had to install RabbitMQ off of their own Debian service to be able to install the required intermediate version to enable the feature flag, before installing the Mantic version. do-release-upgrade can never succeed if you ave rabbitmq-server installed - the package will not be able to be configured.