Fix message queue sample

[ianstanton]

The message queue sample yaml fails to apply with the following error:

2024-04-02T13:05:54.645148Z ERROR reconciling object:reconcile:reconcile:reconcile_cnpg: controller::cloudnativepg::cnpg:1160: Error patching cluster: ApiError: admission webhook "vcluster.cnpg.io" denied the request: Cluster.postgresql.cnpg.io "sample-message-queue" is invalid: spec.resources.requests.memory: Invalid value: "512Mi": Memory request is lower than PostgreSQL `shared_buffers` value: Invalid (ErrorResponse { status: "Failure", message: "admission webhook \"vcluster.cnpg.io\" denied the request: Cluster.postgresql.cnpg.io \"sample-message-queue\" is invalid: spec.resources.requests.memory: Invalid value: \"512Mi\": Memory request is lower than PostgreSQL `shared_buffers` value", reason: "Invalid", code: 422 }) object.ref=CoreDB.v1alpha1.coredb.io/sample-message-queue.default object.reason=reconciler requested retry trace_id=00000000000000000000000000000000 instance_name=sample-message-queue

This PR resolves the issue.

[ChuckHend]

Just a reminder that these yamls are not equivalent to a Stack spec. We can remove this MQ yaml, and the other yamls associated with Stacks to reduce confusion.