clarify 2.6 release notes data plane upgrade requirement

The 2.6 release notes say: "Once your control plane is successfully upgraded, your data plane must be restarted."

AIUI, there is no requirement that your data plane *must* be restarted, and as written it sounds like you need to immediately take an uptime hit while you reboot everything. I suggest removing this sentence or clarifying where it lies in spectrum from "it would be nice" to "mandatory".