Reschedule pending RecalculateVulnerabilitiesOccurrencesUuid jobs

What does this MR do and why?

• Drops old (older than migrations scheduled on 2021-08-18) pending jobs from background_migration_jobs table
• Reschedules pending jobs for RecalculateVulnerabilitiesOccurrencesUuid ran on 2021-08-18

Related to #341195 (closed)

Database review

Remove old, pending records

DELETE FROM background_migration_jobs WHERE class_name = 'RecalculateVulnerabilitiesOccurrencesUuid' AND status = 0 AND created_at <= '2021-08-18';

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/6565/commands/22841

$ bundle exec rails db:migrate
== 20210918201050 RemoveOldPendingJobsForRecalculateVulnerabilitiesOccurrencesUuid: migrating 
== 20210918201050 RemoveOldPendingJobsForRecalculateVulnerabilitiesOccurrencesUuid: migrated (0.0064s)

$ bundle exec rails db:rollback
== 20210918201050 RemoveOldPendingJobsForRecalculateVulnerabilitiesOccurrencesUuid: reverting 
== 20210918201050 RemoveOldPendingJobsForRecalculateVulnerabilitiesOccurrencesUuid: reverted (0.0000s) 

Reschedule pending jobs

$ bundle exec rails db:migrate 
== 20210918202855 ReschedulePendingJobsForRecalculateVulnerabilitiesOccurrencesUuid: migrating 
-- Scheduled 0 RecalculateVulnerabilitiesOccurrencesUuid jobs with an interval of 120 seconds.

The migration is expected to take at least 0 seconds. Expect all jobs to have completed after 2021-09-18 21:01:19 UTC.
== 20210918202855 ReschedulePendingJobsForRecalculateVulnerabilitiesOccurrencesUuid: migrated (0.0114s) 

$ bundle exec rails db:rollback
== 20210918202855 ReschedulePendingJobsForRecalculateVulnerabilitiesOccurrencesUuid: reverting 
== 20210918202855 ReschedulePendingJobsForRecalculateVulnerabilitiesOccurrencesUuid: reverted (0.0000s)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.