Resolve admin plan limits API plans through SystemDefined::Plan

What does this MR do and why?

Switches the admin plan limits API from Admin::PlansFinder to GitlabSubscriptions::SystemDefined::Plan, so it reads limits by plan_name_uid.

Closes #596997 (closed)

How to set up and validate locally

1. Create a personal access token for an admin user with the api and admin_mode scopes.

2. Change a limit on a paid plan:

   curl --request PUT --header "PRIVATE-TOKEN: <token>" \
     "http://gdk.test:3000/api/v4/application/plan_limits?plan_name=premium&maven_max_file_size=100"

   It returns 200 with "maven_max_file_size": 100.

3. In a Rails console, check that the row is keyed by uid and still has plan_id:

   limits = PlanLimits.find_by(plan_name_uid: 5) # premium
   limits.maven_max_file_size # => 100
   limits.plan_id == Plan.find_by(name: 'premium').id # => true

4. Request a plan that has no plans row. free has none unless your GDK has run as GitLab.com, so check first that Plan.exists?(name: 'free') is false:

   curl --header "PRIVATE-TOKEN: <token>" "http://gdk.test:3000/api/v4/application/plan_limits?plan_name=free"

   It returns 404 Plan Not Found, the same as on master.

Database review

I shortened the column lists to "<table>".*.

GET /api/v4/application/plan_limits

current_plan

Before:

SELECT
    "plans".*
FROM
    "plans"
WHERE
    "plans"."name" = 'default'
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163548

After:

SELECT
    1 AS one
FROM
    "plans"
WHERE
    "plans"."plan_name_uid" = 1
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163549

actual_limits

Before:

SELECT
    "plan_limits".*
FROM
    "plan_limits"
WHERE
    "plan_limits"."plan_id" = <plan id>
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163550

After:

SELECT
    "plan_limits".*
FROM
    "plan_limits"
WHERE
    "plan_limits"."plan_name_uid" = 1
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163551

PUT /api/v4/application/plan_limits

current_plan

Before:

SELECT
    "plans".*
FROM
    "plans"
WHERE
    "plans"."name" = 'default'
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163548

After:

SELECT
    1 AS one
FROM
    "plans"
WHERE
    "plans"."plan_name_uid" = 1
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163549

actual_limits

Before:

SELECT
    "plan_limits".*
FROM
    "plan_limits"
WHERE
    "plan_limits"."plan_id" = <plan id>
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163550

After:

SELECT
    "plan_limits".*
FROM
    "plan_limits"
WHERE
    "plan_limits"."plan_name_uid" = 1
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163551

PlanLimits#set_plan_name_uid (plan has no plan_limits row yet)

Before: none.

After:

SELECT
    "plans".*
FROM
    "plans"
WHERE
    "plans"."plan_name_uid" = 1
LIMIT 1

Plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/58555/commands/163552

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