net-mgmt/phpfpm_exporter: Update to 0.6.1

Adapt submitted version (approved by maintainer) to follow Porters
Handbook more closely and switch to go:modules while at it.

Changelog:
https://github.com/bakins/php-fpm-exporter/compare/v0.4.1...v0.6.1

PR: 250787
Reported by: Kamigishi Rei <spambox@haruhiism.net>
Reviewed by: Gasol Wu <gasol.wu@gmail.com> (maintainer)