Drop parent not null constraint from notes

What does this MR do and why?

This constraint was introduced before scheduling the backfill of the notes sharding key. But turns out there are multiple places where we update notes records without using activerecord callbacks. So some records will fail to be updated if they never had a sharding key set

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #444222