Update gem sidekiq-cron from 0.4.4 to 0.6.0 and rufus-scheduler from 3.1.10 to 3.4.0

Description

Let's update gems

• sidekiq-cron from 0.4.4 to 0.6.0
• rufus-scheduler from 3.1.10 to 3.4.0

This is motivated from https://gitlab.com/gitlab-org/gitlab-ce/issues/31274#note_28257378

It also appears that we are using a really old version of that gem, which is not good!

In addition, sidekiq-cron updates caused a problem in the past. (See more details https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10948/diffs#note_28302488 and https://gitlab.com/gitlab-org/gitlab-ce/issues/25160). Let's add a test for catching it.

Proposal

Updateing gems.

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)

/cc @grzesiek @stanhu

mentioned in merge request !10976 (merged)

mentioned in merge request !10948 (merged)

added ~889916 gem update labels

@dosuken123 Thanks for helping out!

assigned to @dosuken123

closed via merge request !10976 (merged)

mentioned in commit a8fb310c