Allow downgrade with message version older than the downgrade version

When downgrading the Kafka version, the operator currently checks that the message format version has to be the same as the broker version which we are downgrading to. This is too strict -> it should check for the version being equal or older than the version we are downgrading to. E.g., when downgrading from 2.6 to 2.5, we should allow the downgrade if the message version is set to 2.4.