Add NOT NULL for sharding key on operations_feature_flags_issues

What does this MR do and why?

Add NOT NULL constraint to operations_feature_flags_issues.project_id column.

• We do not add a validation on the model because we're populating the column using a "BEFORE INSERT OR UPDATE" trigger.
• We have finalized the backfill migration for this column.

The sharding key will be used to determine what Organization the row belongs to.

References

• Organization Isolation and Sharding Keys: https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/organization/isolation/
• Sharding Key (Cells) Development Guide: https://docs.gitlab.com/ee/development/cells/#defining-a-sharding-key-for-all-cell-local-tables
• Related to #493768

MR acceptance checklist

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How to set up and validate locally

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db/docs/operations_feature_flags_issues.yml

@@ -8,14 +8,6 @@ description: https://docs.gitlab.com/ee/operations/feature_flags.html#feature-fl
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/32876
 milestone: '13.1'
 gitlab_schema: gitlab_main_cell
-desired_sharding_key:
-  project_id:
-    references: projects
-    backfill_via:
-      parent:
-        foreign_key: feature_flag_id
-        table: operations_feature_flags
-        sharding_key: project_id
-        belongs_to: feature_flag
-desired_sharding_key_migration_job_name: BackfillOperationsFeatureFlagsIssuesProjectId
 table_size: small
+sharding_key:
+  project_id: projects