Add NOT NULL for sharding key on epic_user_mentions

What does this MR do and why?

Add NOT NULL constraint to epic_user_mentions.group_id column.

• No presence: true check is added in the model because we're also populating the column with a "BEFORE INSERT OR UPDATE" trigger.
  • Objects created in rails are allowed to have a project_id=nil, the trigger would populate it before being committed.
• We use async validation for this because the table is fairly big and the non-async version exceeds the timing guidelines.
• No follow up issue is created to finalize the async validation because this will be tracked in #493768 (closed)
• The backfill migration for this column is completed and has been finalized.

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 (closed)

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