release: Enforce success on Fedora 26, drop Fedora 25
release: Enforce success on Fedora 26, drop Fedora 25

We don't regularly release to Fedora 25 any more, so drop koji build.

Stop allowing failures on Fedora 26 as that is now our primary target.
See https://github.com/cockpit-project/cockpit/pull/6602

stefwalter/cockpituous,cockpit-project/cockpituous,stefwalter/cockpituous,cockpit-project/cockpituous