For next 2 milestones adds csouthard as tech advisor

Why is this change being made?

With recent reduction in force actions, we no longer have a dedicated role to support technical aspects of the release post. @csouthard has volunteered this several times before, and is signing up to support the process at least in the short-term while processes are in flux.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
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