Support updating plan limit enforcement_limit

What does this MR do and why?

Adds API support for updating PlanLimit#enforcement_limit

Allowing admins to update a plan limit's enforcement_limit value so that they can be controlled via an admin UI as part of https://gitlab.com/groups/gitlab-org/-/epics/9173

Refs https://gitlab.com/gitlab-org/gitlab/-/issues/385737

How to set up and validate locally

curl --request PUT \
  --url 'http://gdk.test:3000/api/v4/application/plan_limits?plan_name=free&enforcement_limit=10' \
  --header 'PRIVATE-TOKEN: your-admin-pat-here' 

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.