Document lack of 1.18 support (#931)

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.

Author: coro

README.md

@@ -37,7 +37,7 @@ under `site/kubernetes`.
 
 ## Supported Versions
 
-The operator deploys RabbitMQ `3.8.21` by default, and supports versions from `3.8.8` upwards. The operator requires Kubernetes `1.18` or newer.
+The operator deploys RabbitMQ `3.8.21` by default, and supports versions from `3.8.8` upwards. The operator requires Kubernetes `1.19` or newer.
 
 ## Versioning