Document lack of 1.18 support

[coro]

This closes #930

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

Summary Of Changes

Kubernetes 1.18 is no longer supported by the operator, as of v1.11.0. This is due to the feature gate ServiceAppProtocol being disabled by default in 1.18; the feature is needed by the RabbitmqCluster service.

Note: I also edited the v1.11.0 release notes to document the drop of support.

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