zed: prettify slack notification message

This converts the body of a ZED slack notification from
plain text to code block style to help with readability.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: René Wirnata <rene.wirnata@pandascience.net>
Closes #17610