www/varnish_exporter: Update to 1.5.1

While here:

• convert to go:modules
• pet portclippy

Changes: https://github.com/jonnenauha/prometheus_varnish_exporter/releases/tag/1.5.1

PR: 240782
Submitted by: Žygis Škulteckis <ports@zx23.net> (maintainer)
Approved by: tz (mentor)
Differential Revision: https://reviews.freebsd.org/D21899