Remove deprecated storefront_access_token positional parameter from Storefront client

This removes the deprecation introduced in PR #1302 (v14.1.0) in preparation for the v15.0.0 major release. Users must now use named parameters (private_token: or public_token:) instead of passing the token as a positional argument.

Updated Storefront client to raise ArgumentError when no token is provided, updated tests to use named parameters, and added breaking change documentation with migration examples.

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