Resolve admin plan limits by plan_name_uid

What does this MR do and why?

Resolves the plan in the admin plan-limits flow by plan_name_uid instead of plan_id:

• Admin::PlanLimitsController looks up the plan with Plan.find_by_plan_name_uid! only; the plan_id fallback and its permit entry are removed.
• The inert plan_id hidden field is dropped from the CI/CD and package-registry settings partials.

The plans table is being replaced by an application-defined model, so plan references must stop depending on its environment-specific row ids; plan_name_uid is the stable identifier that survives the table's removal. This is safe now because the form has rendered and submitted plan_name_uid since !235913 (merged) shipped in 19.2, and an unresolvable uid raises RecordNotFound (404) instead of matching the wrong plan.

The remaining Phase 2 items (model write path, admin REST API, migration helper) are tracked in the issue. No schema change.

Issue: #596997 (closed)

How to set up and validate locally

1. Run the touched specs:

bin/rspec spec/controllers/admin/plan_limits_controller_spec.rb \
          spec/views/admin/application_settings/_ci_cd.html.haml_spec.rb \
          spec/views/admin/application_settings/_package_registry.html.haml_spec.rb

2. Go to Admin → Settings → CI/CD, expand Continuous Integration and Deployment, and scroll to the CI/CD limits form at the bottom of the section.

3. Change a limit and save. Confirm the save succeeds and the POST /admin/plan_limits parameters in log/development.log include plan_limits[plan_name_uid] and no plan_id.

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