net-mgmt/pushgateway: Update to 1.3.1

• Removed code to convert the legacy v0.5--v0.9 storage format

Changelog: https://github.com/prometheus/pushgateway/releases

PR: 251229
Submitted by: Gabriel Dutra <0xdutra@gmail.com> (maintainer)
Approved by: arrowd (mentor)
MFH: 2021Q1
Differential Revision: https://reviews.freebsd.org/D27795