Move cert-manager into `core`

[mikebell]

Background

Move certmanager and associated configuration into the core terraform infrastructure

This may need to be done at the same time as #5320. Can we decouple this?

Proposed user journey

Approach

Which part of the user docs does this impact

Communicate changes

• [ ] post for #cloud-platform-update
• [ ] Weeknotes item
• [ ] Show the Thing/P&A All Hands/User CoP
• [ ] Announcements channel

Questions / Assumptions

Definition of done

• [ ] readme has been updated
• [ ] user docs have been updated
• [ ] another team member has reviewed
• [ ] smoke tests are green
• [ ] prepare demo for the team

Reference

How to write good user stories