Bump default RMQ version

[ChunyiLyu]

This closes #

Note to reviewers: remember to look at the commits in this PR and consider if they can be squashed

Summary Of Changes

Bump default RMQ version when spec.image is not set from 3.10.2 to latest 3.11.x.

Additional Context

Local Testing

Please ensure you run the unit, integration and system tests before approving the PR.

To run the unit and integration tests:

$ make unit-tests integration-tests

You will need to target a k8s cluster and have the operator deployed for running the system tests.

For example, for a Kubernetes context named dev-bunny:

$ kubectx dev-bunny
$ make destroy deploy-dev
# wait for operator to be deployed
$ make system-tests