net-mgmt/pushgateway: Update to 1.4.0

Experimental new feature:
Add TLS and basic authentication to HTTP endpoints.

Changelog: https://github.com/prometheus/pushgateway/releases/tag/v1.4.0

PR: 253007
Submitted by: Gabriel Dutra <0xdutra@gmail.com> (maintainer)
Approved by: arrowd, tcberner (mentors, implicit)