security/zeek: Limit portscout to even long term support release versions
https://github.com/zeek/zeek/releases Zeek 3.0.x is the Long-Term Support release, receiving bug fixes until at least October 2020 while Zeek 3.1.x is the current feature release, receiving bug fixes until approximately July 2020 when the 3.2.x release series begins.
Approved by: matthew (mentor, implicit)