Fix populate latest pipeline ids background migration

What does this MR do?

This MR fixes the constant namespace resolution issue for one of the existing background migrations. Also, encapsulates the logic for each project being migrated into a begin&rescue block to prevent complete failure just in case the logic fails for a project.

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2699384 https://sentry.gitlab.net/gitlab/gitlabcom/issues/2699548

Related to #271408 (closed), !63451 (merged)

Database review

This MR marks one of the existing migrations as no-op and introduces a new one to re-run the same logic.

rake db:migrate:up VERSION=20210706142819

== 20210706142819 ReScheduleLatestPipelineIdPopulation: migrating =============
-- Scheduled 1 PopulateLatestPipelineIds jobs with a maximum of 100 records per batch and an interval of 120 seconds.

The migration is expected to take at least 120 seconds. Expect all jobs to have completed after 2021-07-06 14:40:18 UTC."
== 20210706142819 ReScheduleLatestPipelineIdPopulation: migrated (0.0679s) ====

rake db:migrate:down VERSION=20210706142819

== 20210706142819 ReScheduleLatestPipelineIdPopulation: reverting =============
== 20210706142819 ReScheduleLatestPipelineIdPopulation: reverted (0.0000s) ====

Does this MR meet the acceptance criteria?

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