fix: Cart_Mutation:prepare_attributes made public

[kidunot89]

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What does this implement/fix? Explain your changes.

• Makes the prepare_attributes of the Cart_Mutation class a public function so it can be used in WooGraphQL Pro and other mutations utilizing the cart functionality.

Does this close any currently open issues?

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Any relevant logs, error output, GraphiQL screenshots, etc?

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Any other comments?

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Where has this been tested?

• WooGraphQL Version: …
• WPGraphQL Version: …
• WordPress Version:
• WooCommerce Version: