Switch varnish exporter to otto-de fork

[TheMeier]

The varnish exporter project at https://github.com/jonnenauha/prometheus_varnish_exporter looks orphaned.
Last release and last code change was in December 2021.

[TheMeier]

There is a fork https://github.com/otto-de/prometheus_varnish_exporter but that fork also has not been updated in a while and has no releases yet.
otto-de/prometheus_varnish_exporter#3

[AljoschaDembowsky]

We started to actively maintaining our fork 👍 Feel free to check out our first release: https://github.com/otto-de/prometheus_varnish_exporter/releases/tag/v1.7.0 - we also provide container images here

[TheMeier]

@AljoschaDembowsky I had a look into it. Your fork changed the package names, the binary name and the archive layout. That makes it really hard to adopt it, and extra hard to make it so upgrades can be done properly. :(