Put varnish 6.0 into separate branch

Varnish has unusual release cycle where 6.0 is lts and 6.1-6.6 are deprecated.
Currently, 6.0.12 is considered outdated vecause there are newer 6.x releases
but in fact it should be considered legacy as there are no newer 6.0.x releases.
Fix that by putting 6.0 into its own branch.

Issue: #235
See also: https://varnish-cache.org/docs/index.html

950.split-branches.yaml

@@ -384,6 +384,7 @@
 - { name: unifi,                                          setbranchcomps: 1 }
 
 - { name: varnish,                                        setbranchcomps: 1 }
+- { name: varnish,           releq: "6.0",                setbranch: "6.0lts", maintenance: true }
 
 - { name: virtualbox,                                     setbranchcomps: 2 }