Renamed `isStripeConfigured` to `arePaidMembersEnabled` in Portal (#28299)

no issue

This change should not affect behaviour.

- the field `site.is_stripe_configured` was a client-side alias for
  `site.paid_members_enabled` (a calculated setting backed by
  `SettingsHelpers.arePaidMembersEnabled()` server-side, which means
  members enabled AND Stripe connected), so the old name was misleading
- removed the alias creation in `transformApiSiteData` and updated every
caller to read `paid_members_enabled` / `arePaidMembersEnabled` directly
  account-welcome, getAvailablePrices) keeps the same behavior