Updates tw-milestone-plan.md for GitLab Dedicated

Adds stage and group for GitLab Dedicated to the TW milestone plan template.

There is currently no group label for Switchboard, as it's a team that is part of the Dedicated group. However, in the GitLab Dedicated Issue Tracker where Switchboard work is tracked, the following labels are used: ~team::switchboard and ~component::switchboard.

Switchboard Technical Writing board

Relates to: Technical Writing milestone plan for 16.5 (#919 - closed) and https://gitlab.com/gitlab-org/technical-writing/-/issues/887+

Stage devopsplatforms

• groupdedicated - Link to board or filter