Add sharding key to board_assignees

What does this MR do and why?

Add group or project as sharding key. Backfill all records in a post deploy migration as the table is very small (7466 records)

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• https://docs.gitlab.com/ee/development/cells/index.html#defining-a-sharding-key-for-all-cell-local-tables

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #514603 (closed)