Update guidance on iterations & milestones

Our usage of tracking sprints in Gitlab has changed from using Milestones to Iterations.

This is our existing documentation on milestones: https://guidebook.devops.uis.cam.ac.uk/en/latest/workflow/gitlab/#milestones

Additionally, it would be good to set a standard on naming iterations that does not include a space character, as these are difficult to filter on in Gitlab's UI. Suggested notation e.g. "2022-A".