Add Slack notifications for Container Registry PDM pipeline execution

Problem statement

As part of the Container Registry post deployment migrations (PDMs) automation effort (epic: gitlab-org&17933), we need to implement proper notifications to keep relevant teams informed about PDM execution status.

This issue is a follow-up to #21259 (closed).

Proposal

Implement Slack notifications for the Container Registry PDM pipeline to notify relevant channels about:

• Pipeline start/completion status
• Success/failure notifications
• Environment-specific execution updates

Target Channels

• #announcements
• #g_container-registry

Notification Types

• Pipeline Start: When PDM execution begins
• Environment Completion: When PDMs complete successfully in each environment (pre, staging, production)
• Pipeline Failure: When any environment fails, with relevant links to failed jobs

Acceptance Criteria

• Slack notifications are sent to #announcements channel
• Slack notifications are sent to #g_container-registry channel
• Notifications include relevant context (environment, pipeline URL, status)
• Success notifications are sent when PDMs complete in each environment
• Failure notifications include error details and troubleshooting links