Merge branch '514603-add-column-and-backfill' into 'master'

Add sharding key to board_assignees

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/178949



Merged-by: Mario Celi <mcelicalderon@gitlab.com>
Approved-by: Gosia Ksionek <mksionek@gitlab.com>
Approved-by: Krasimir Angelov <kangelov@gitlab.com>
Reviewed-by: Gosia Ksionek <mksionek@gitlab.com>
Reviewed-by: Krasimir Angelov <kangelov@gitlab.com>
Reviewed-by: Mario Celi <mcelicalderon@gitlab.com>