net-mgmt/check_mk_agent: Update 2.3.0p16 => 2.4.0p20

Commit log:
https://github.com/Checkmk/checkmk/commits/7e6a4f8/agents/check_mk_agent.freebsd

Upstream hasn't created tags for new versions since 2.4.0p12, so we use
the hash.

PR: 292480
Approved by: Ian Valentine <ivalentine@arizona.edu> (maintainer, timeout 3 weeks)
Co-authored-by: Lukas Engelhardt <lukas.engelhardt@gmx.de>