Define a sharding key for note_diff_files table

Table note_diff_files should meet the guidelines for sharding so it can be linked to an Organization: https://docs.gitlab.com/development/organization/#defining-a-sharding-key-for-all-organizational-tables

Sharding key considerations

Important

Marking all tables that were blocked by the notes backfill with this message. Please read through #444222 (comment 2765183660) as it might affect the structure of the sharding key for this table

Completion Criteria

When the work is complete, the new column should fulfil the following criteria:

• has no default
• is not nullable
• has a foreign key to organizations