Stop using LockCol in registrations table (#7935)

Alter the `LockCol` column to have a default value, so we can omit it
from `INSERT`s.

Part of #7934

sa/database.go

@@ -265,7 +265,6 @@ func (log *SQLLogger) Printf(format string, v ...interface{}) {
 func initTables(dbMap *borp.DbMap) {
 	regTable := dbMap.AddTableWithName(regModel{}, "registrations").SetKeys(true, "ID")
 
-	regTable.SetVersionCol("LockCol")
 	regTable.ColMap("Key").SetNotNull(true)
 	regTable.ColMap("KeySHA256").SetNotNull(true).SetUnique(true)
 	dbMap.AddTableWithName(issuedNameModel{}, "issuedNames").SetKeys(true, "ID")

sa/db-next/boulder_sa/20250110000000_NullRegistrationsLockCol.sql

@@ -0,0 +1,10 @@
+
+-- +migrate Up
+-- SQL in section 'Up' is executed when this migration is applied
+
+ALTER TABLE `registrations` ALTER COLUMN `LockCol` SET DEFAULT 0;
+
+-- +migrate Down
+-- SQL section 'Down' is executed when this migration is rolled back
+
+ALTER TABLE `registrations` ALTER COLUMN `LockCol` DROP DEFAULT;