Remove deprecated ShopifyAPI::Webhooks::Handler interface

In preparation for the next major version, the deprecated Handler interface has been removed.

Apps must migrate to WebhookHandler which provides webhook_id and api_version fields in addition to topic, shop, and body. This enables apps to properly handle duplicate webhook events and version-specific processing. All tests and documentation have been updated to reflect the new interface.

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Co-Authored-By: Claude <noreply@anthropic.com>