Add `updated_at` params to various endpoints in our REST API

Problem

Some of the current rest APIs are missing an updated_at field, which means API users always have to query ALL data rather than just what has changed since they last checked. This creates a huge extra load on Gitlab, and slows down any reporting options (to the point that we have some funcitonality we just won't build as swe are worried about the speed and platform impact)

Proposal

Add updated_at attribute (where missing) and sort/filter params by updated_at to:

• https://docs.gitlab.com/ee/api/group_milestones.html
• https://docs.gitlab.com/ee/api/milestones.html
• https://docs.gitlab.com/ee/api/projects.html
• https://docs.gitlab.com/ee/api/groups.html
• https://docs.gitlab.com/ee/api/group_iterations.html
• https://docs.gitlab.com/ee/api/iterations.html
• https://docs.gitlab.com/ee/api/releases/