Bump rufus-scheduler to 3.5.0

It turned out there is a problem with the current code that we can't bump rufus-scheduler gem to 3.5.0.

We should resolve the problem to keep the gem up-to-date.

---

Old issue title: unneeded check in lib/gitlab/ci/cron_parser.rb

       def try_parse_cron(cron, cron_timezone)
         cron_line = Rufus::Scheduler.parse("#{cron} #{cron_timezone}")
         cron_line if cron_line.is_a?(Rufus::Scheduler::CronLine)
       rescue
         # noop
       end

The parser in rufus-scheduler 3.4.x and 3.5.x throws argument error if the cron line can not be parsed. The problem is when you want to push rufus-scheduler to 3.5.x, the parse method returns a Fugit::Cron now, which breaks the assumption in the code.

Would removing the is_a? check and relying on the exception be ok?