Add not null contraint to requirements.issue_id with validate: false

What does this MR do and why?

First step to add not null constraint to requirements.issue_id column

• Reschedule background migration to populate requirements.issue_id when nil. There are no invalid records on production, but it may not be the case on self managed.
• Add NOT NULL constraint to requirements.issue_id column with validate: false.

related to #348188 (closed)

Migration output

== 20220506123922 SchedulePopulateRequirementsIssueId: migrating ==============
-- Scheduled 0 MigrateRequirementsToWorkItems jobs with a maximum of 30 records per batch and an interval of 120 seconds.

The migration is expected to take at least 0 seconds. Expect all jobs to have completed after 2022-05-06 17:07:26 UTC."
== 20220506123922 SchedulePopulateRequirementsIssueId: migrated (0.0078s) =====

== 20220506124021 AddNotNullConstraintWithoutValidationToRequirementsIssueId: migrating 
-- current_schema()
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0003s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE requirements\nADD CONSTRAINT check_ac19122290\nCHECK ( issue_id IS NOT NULL )\nNOT VALID;\n")
   -> 0.0006s
== 20220506124021 AddNotNullConstraintWithoutValidationToRequirementsIssueId: migrated (0.0082s) 

MR acceptance checklist

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