Add NOT NULL constraint to notes.namespace_id

As part of &12795 we need to add sharding keys to all our tables. notes already has project_id from way-back, however sharding keys are required to be NOT NULL so we will need to embark on the multi-release process of migrating that column, as outlined here: https://docs.gitlab.com/ee/development/database/not_null_constraints.html#add-a-not-null-constraint-to-an-existing-column

This will delay completion of adding sharding keys to tables that want to backfill their own sharding keys from notes.

Order of Work

• Delete abuse_notes records
• Add not null constraint on notes.namespace_id
• Set notes.namespace_id as the sharding key for notes