FIX: Set null high_priority columns to false in high priority notification migration (PR #9335)

Some DBs failed on this migration, possibly due to new records being added, with the error

ActiveRecord::NotNullViolation: PG::NotNullViolation: ERROR:  column "high_priority" contains null values

GitHub

The title of this pull request changed from “FIX: Set null high_priority columns to false in high priority migration” to "FIX: Set null high_priority columns to false in high priority notification migration