Introduce retry mechanism

[leochab]

Context

When setting up checkers, some of them may return false positives for various reasons.

Solution

Create a retry mechanism for each checker.

• If a checker does not need to trigger any notification, its behaviour remains unchanged.
• If a checker would need to trigger a notification, we now:
  • Verify whether the checker should be retried. If so, relaunch it; otherwise, trigger the notification.
  • Each checker can be retried a maximum of three times.
  • If any of the retried job does not return any result, meaning it would not send any notification, stop the process.

Extra

Use this PR to update the required Ruby version and perform some code cleaning.