Drop contact column from registrations table (letsencrypt#8201)

Drop the contact column from the Registrations table.

Part of letsencrypt#8199

Author: aarongable

sa/db-next/boulder_sa/20250520000000_DropRegistrationsContact.sql

@@ -0,0 +1,9 @@
+-- +migrate Up
+-- SQL in section 'Up' is executed when this migration is applied
+
+ALTER TABLE `registrations` DROP COLUMN `contact`;
+
+-- +migrate Down
+-- SQL section 'Down' is executed when this migration is rolled back
+
+ALTER TABLE `registrations` ADD COLUMN `contact` varchar(191) CHARACTER SET utf8mb4 DEFAULT '[]';