sysutils/ping_exporter: Update to 1.2.1
Release notes:
https://github.com/czerwonk/ping_exporter/releases/tag/v1.2.1
While here, add a pkg-message for a breaking change in 1.2.0.
Reviewed by: arrowd
Approved by: arrowd (ports)
Differential Revision: https://reviews.freebsd.org/D57126