Let's add MinVer and on successful builds generate a release.

Background

As discussed in #223 comments (zalando-incubator/kopf#223 (comment) and below), there is one type of handlers is missing: when an operator restarts, it should process all the objects that existed before, but were neither updated, nor deleted.

Originally, this was a technical solution for @kopf.on.resume() handlers in #96. But then it was

As a followup to #696 we should deal with Ruby 2.7 deprecation warnings. Most common issue is:
Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call.

Fairly easy to fix, I suppose.