jobs/seed-github-ci: fix branch rescan trigger

I think I got it right this time. The docs for that key specifically
mention SCM re-indexing and push notification fallback.

Author: jlebon

jobs/seed-github-ci.Jenkinsfile

@@ -43,9 +43,11 @@ node { repos.each { repo ->
                 discardOldItems()
             }
             triggers {
-                // manually rescan once a day; this is important so that it
-                // picks up on deleted branches/PRs which can be cleaned up
-                cron((60 * 24).toString())
+                periodicFolderTrigger {
+                    // manually rescan once a day; this is important so that it
+                    // picks up on deleted branches/PRs which can be cleaned up
+                    interval((60 * 24).toString())
+                }
             }
             // things which don't seem to have a nice DSL :(
             configure {