Prefer to use /cache over external cache

What does this MR do?

Use disk cache instead of external cache.

This also adds a disclaimer how to make changes to cache and artifacts. Since this has significant implication it should be approved either by ~"CI/CD" or ~Edge team.

@stanhu Is that a good place to enforce that rule?

Why was this MR needed?

This is to solve my woes of https://gitlab.com/gitlab-com/infrastructure/issues/2724.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-com/infrastructure/issues/2724

cc @stanhu @rymai