Update Access group code review / feature flag process

Why is this change being made?

Recently in Access, 3 production incidents were caused that could have been contained by using a feature flag. These incidents were related to the work done on Linear Namespaces, where we were initially very cautious about rolling out gradually and able to identify issues in advance. Outside of Linear Namespaces, all Access work is far-reaching and changes have the potential to cause disruption across all members. Unfortunately, this leaves little room for error and may require special circumstances.

This is documentation for #12276 (closed)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
• 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.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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