refactor(notification): use new queue package (#9360)

# Which Problems Are Solved

The recently introduced notification queue have potential race conditions.

# How the Problems Are Solved

Current code is refactored to use the queue package, which is safe in
regards of concurrency.

# Additional Changes

- the queue is included in startup
- improved code quality of queue

# Additional Context

- closes https://github.com/zitadel/zitadel/issues/9278

internal/notification/handlers/queue.go

@@ -0,0 +1,13 @@
+package handlers
+
+import (
+	"context"
+
+	"github.com/riverqueue/river"
+
+	"github.com/zitadel/zitadel/internal/queue"
+)
+
+type Queue interface {
+	Insert(ctx context.Context, args river.JobArgs, opts ...queue.InsertOpt) error
+}