Update Container Registry Replication configurations in GitLab Charts and Omnibus

What does this MR do?

Update Registry notification configurations in GitLab Charts:

1. Move registry.notificationSecret to global.registry.notificationSecret - https://docs.gitlab.com/charts/charts/globals.html#configure-registry-settings
2. Remove container_registry_notification_token and its usage, it's not needed for Replication
3. Update GitLab Charts configuration for global.geo.registry

Guidance for upgrade for Registry replication in Geo

Container Registry Replication for Geo was updated to follow the latest GitLab guidance.

For Omnibus Geo setups:

• Remove container_registry_notification_token. Only container_registry_token value is needed to be specified in both Geo sites.

For Cloud Native Hybrid Geo setups:

• Remove container_registry_notification_token. Only container_registry_token value is needed to be specified in both Geo sites.
• After deploying to environments using this GET version, it is recommended to push previously pushed containers once more to ensure that all containers are in place.

Related issues

Closes Container Registry Replication doesn't work in ... (#670 - closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
    • Test Geo setup - migrating from old GET code to the code from this branch
    • No geo: Test setup without Registry enabled
    • Test Geo setup - new CNH envs with Replication enabled
  • Documentation created/updated in the same MR.