Enable slack deployment notifications

This differs from the `notify_slack` setting, which is more like
`alert_slack_on_failed_deployments`.

I'll maybe rename this separately...

Related:
communitiesuk/funding-service-design-workflows#265

.github/workflows/copilot_deploy.yml

@@ -475,10 +475,12 @@ jobs:
       AWS_ACCOUNT: ${{ secrets.AWS_ACCOUNT }}
       SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
       SLACK_NOTIFICATION_CHANNEL_ID: ${{ secrets.SLACK_NOTIFICATION_CHANNEL_ID }}
+      SLACK_DEPLOYMENTS_CHANNEL_ID: ${{ secrets.SLACK_DEPLOYMENTS_CHANNEL_ID }}
     with:
       environment: prod
       app_name: form-runner-adapter
       run_db_migrations: false
       image_location: ghcr.io/communitiesuk/funding-service-design-form-runner-adapter:sha-${{ github.sha }}
       notify_slack: true
+      notify_slack_on_deployment: true