add timeline for chg man control family remediation

Why is this change being made?

document in handbook the timeline discussed during the change management proposal discussion

https://gitlab.com/gitlab-com/gl-security/compliance/compliance/-/issues/1765

https://docs.google.com/document/d/1O2dkI7P_G79weY1nIVXP-b0EbsUNpnqqbDNreKxgyts/edit

Author Checklist

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