Update db backup retention details

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).