Update Handbook for May 2023 GitLab.com Availability

Why is this change being made?

To follow the process as documented here: https://about.gitlab.com/handbook/engineering/infrastructure/team/reliability/general.html#monthly-availability-updates

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)

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