Update DB backup retention details

Why is this change being made?

Updates db backup retention details to match upcoming changes https://gitlab.com/gitlab-com/gl-infra/dbre/-/issues/90

• Reduce standard duration from 14 days to 7 days.
• Remove nearline tier, going directly to coldline instead.
• Reduce coldline duration from 120 days to its minimum 90 days.

Anticipated results:

• Reduce our recovery point window from 120 days to 90 days.
• Reduce our storage cost from $10175/month to $6670/month, saving $3500/month ($42K/year).

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• 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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• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • 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 on the page being edited
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  • The when to get approval handbook section explains the workflow in more detail
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
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    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

Commits

• Remove reference to diagrams that don't exist and update DB backup retention to match current policy

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