Add `cluster_upgrade_SUITE` to check mixed-version upgrades (backport #11967) (backport #11968)#11969

the-mikedavis merged 1 commit intov3.13.xrabbitmq/rabbitmq-server:v3.13.xfrom

mergify/bp/v3.13.x/pr-11968rabbitmq/rabbitmq-server:mergify/bp/v3.13.x/pr-11968Copy head branch name to clipboard