Allow updating shared_runners_minutes_limit on admin Namespace API

As part of https://gitlab.com/gitlab-com/customers-gitlab-com/issues/124, this MR enables customers.gitlab.com to update the Namespace#plan and Namespace#shared_runners_minutes_limit with a single request the admin API PUT /namespaces/:id.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/customers-gitlab-com/issues/124, https://gitlab.com/gitlab-org/gitlab-ee/issues/1909