-Xmx for pods whould not be determined from memory limit but memeory request

Following the discussion on Slack, we should use the memory request and not limit to determine the default -Xmx setting. The memeory limit might never be granted to the pod, so using it to determine -Xmx can lead to crashloops of KAfka when it cannot allocate the memeory at all.

This will impact everyone who use the old defaults. It should be mentioned in release notes.

Created by Strimzi#2203