security-command-center: no code sample to receive notifications for v2 API

## In which file did you encounter the issue?

* v1 code sample for receiving notifications: https://github.com/GoogleCloudPlatform/java-docs-samples/blob/611b32be/security-command-center/snippets/src/main/java/NotificationReceiver.java
* All v2 code samples for notifications: https://github.com/GoogleCloudPlatform/java-docs-samples/tree/611b32be/security-command-center/snippets/src/main/java/vtwo/notifications

## Did you change the file? If so, how?

No

## Describe the issue

In #9276, we added code samples that show how to manage notifications with the Security Command Center v2 API. However, we didn't add a v2 version of [`NotificationReceiver.java`](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/611b32be249e3ed03fb523168c17a22013eb6fae/security-command-center/snippets/src/main/java/NotificationReceiver.java).

We need to create a v2 version of this sample, with the region tag `securitycenter_receive_notifications_v2`.

cc: @owenhuyn