Update documentation to explicitly state we should not be putting migrations behind feature flags (#48663)

pkg/services/sqlstore/migrations/migrations.go

@@ -13,6 +13,9 @@ import (
 // 1. Never change a migration that is committed and pushed to main
 // 2. Always add new migrations (to change or undo previous migrations)
 // 3. Some migrations are not yet written (rename column, table, drop table, index etc)
+// 4. Putting migrations behind feature flags is no longer recommended as broken
+//    migrations may not be caught by integration tests unless feature flags are
+//    specifically added
 
 type OSSMigrations struct {
 }