sysutils/graylog: update to 4.2.6

• minor UI denial-of-service issues in markdown parser

ChangeLog: https://docs.graylog.org/docs/changelog#graylog-426

Reported by: portscout
Sponsored by: SkunkWerks, GmbH
MFH: 2022Q1

(cherry picked from commit 8aeb1e33ac3559e4bafbb178b0278dbd6790dacf)