Cleaned up the gift subscriptions feature flag (#27862)

closes https://linear.app/tryghost/issue/BER-3646/

- removed the `giftSubscriptions` labs flag and all backend / frontend
code guards
- dropped the now-redundant `hasGiftSubscriptions` Portal helper and
simplified `canPurchaseGift` to a Stripe check
- removed the corresponding disabled-branch unit/integration/e2e tests
and updated the admin config snapshot

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Co-authored-by: Sag <guptazy@gmail.com>