fix(notifications): bring back legacy notification handling (#9015)

# Which Problems Are Solved

There are some problems related to the use of CockroachDB with the new
notification handling (#8931).
See #9002 for details.

# How the Problems Are Solved

- Brought back the previous notification handler as legacy mode.
- Added a configuration to choose between legacy mode and new parallel
workers.
  - Enabled legacy mode by default to prevent issues.

# Additional Changes

None

# Additional Context

- closes https://github.com/zitadel/zitadel/issues/9002
- relates to #8931

internal/notification/handlers/notification_worker.go

@@ -43,6 +43,7 @@ type NotificationWorker struct {
 }
 
 type WorkerConfig struct {
+	LegacyEnabled         bool
 	Workers               uint8
 	BulkLimit             uint16
 	RequeueEvery          time.Duration
@@ -97,6 +98,9 @@ func NewNotificationWorker(
 }
 
 func (w *NotificationWorker) Start(ctx context.Context) {
+	if w.config.LegacyEnabled {
+		return
+	}
 	for i := 0; i < int(w.config.Workers); i++ {
 		go w.schedule(ctx, i, false)
 	}