Add note about GitLab Dedicated incident updates

#### CTO Updates

The Infrastructure Leadership should make updates to the #cto Slack channel for all S1 incidents at incident open, significant status change (such as Investigating to Fixing), and at incident resolution. The updates should follow the standard incident.io Summary format and can typically be copy/pasted from the incident summary.

The Infrastructure Leadership should make updates to the #cto Slack channel for all S1 incidents at incident open, significant status change (such as Investigating to Fixing), and at incident resolution for both GitLab.com and GitLab Dedicated incidents.

The updates should follow the standard incident.io Summary format used for GitLab.com and can typically be copy/pasted from the incident summary.

You can ask the @incident Slack bot to draft you an executive summary for the incident if the existing summary is insufficient.

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