Update documentation to explicitly state we should not be putting migrations behind feature flags (#48663) (#48666)

(cherry picked from commit 38fc0c68e4)

Co-authored-by: Jeff Levin <jeff@levinology.com>

contribute/architecture/backend/database.md

@@ -99,6 +99,8 @@ To add a migration:
 - In the `AddMigrations` function, find the `addXxxMigration` function for the service you want to create a migration for.
 - At the end of the `addXxxMigration` function, register your migration:
 
+> **NOTE:** Putting migrations behind feature flags is no longer recommended as it may cause the migration skip integration testing.
+
 [Example](https://github.com/grafana/grafana/blob/00d0640b6e778ddaca021670fe851fe00982acf2/pkg/services/sqlstore/migrations/migrations.go#L55-L70)
 
 ### Implement `DatabaseMigrator`

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 {
 }