fix: use common sql syntax for dropping null constraint on birthday field

private/minis-backend/src/main/kotlin/de/walamana/plugins/Databases.kt

@@ -30,7 +30,7 @@ fun Application.configureDatabases() {
         SchemaUtils.create(Gottesdienste)
         SchemaUtils.create(Marks)
         // TODO Add proper versioning and migration for database
-        exec("ALTER TABLE MINISTRANTEN ALTER COLUMN birthday BIGINT NULL;")
+        exec("ALTER TABLE MINISTRANTEN ALTER COLUMN birthday DROP NOT NULL;")
     }
 }