Backport of 'Remove check_f02a3f53bf not null constraint'

What does this MR do and why?

This MR removes not null constraint check_f02a3f53bf from issue_tracker_data table. It's been reported in #571338 (closed) that for some users there are invalid rows in this table, preventing GitLab from upgrading.

It looks like there are some rows in the table that have both org_id & project_id/group_id set, making the not null constraint invalid. We did not have any evidence of such invalid rows on .com, so I thought we're in the clear to proceed with adding the not null constraint.

I am removing the constraint for time being and picking this change in 18.4 to unblock upgrades and will create a data backfill + re-introduction of the constraint in the future MR.

References

#571338 (closed) #558260 (closed)

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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