Add NOT NULL constraint to operations_scopes

What does this MR do and why?

Adds NOT NULL constraint on sharding key column project_id on table: operations_scopes.

This merge request is the last step towards transforming the desired sharding key into a sharding key.

This involves the following changes:

• Adding a not null constraint for the sharding key columns (project_id).
• Setting project_id as sharding_key of operations_scopes.

We are skipping model-level validations when adding NOT NULL for now: !189213 (comment 2480466972)

The backfilling migrations were finished and finalized:

• https://gitlab.com/gitlab-org/gitlab/blob/79dad1c0962aef3c803d940e5ffa90ce43e7dc48/db/docs/batched_background_migrations/backfill_operations_scopes_project_id.yml

References

• !189213 (comment 2480466972)
• https://docs.gitlab.com/ee/development/database/multiple_databases.html#defining-a-sharding-key-for-all-cell-local-tables
• https://docs.gitlab.com/development/database/not_null_constraints/

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