Add DNS record update instructions for gitlab.com subdomains

Due to team responsibility changes in https://gitlab.com/gitlab-com/gl-security/identity/eng/issue-tracker/-/issues/2#note_1682069589, we are updating domain records from white glove service by IT Infrastructure (that no longer exists) to self-service using Terraform MRs that can be merged by any team member in Infrastructure.

This moves the steps from the legacy IT Infrastructure runbook into the existing Reliability runbook that can be self serviced by team members.