"Coupon_Connection_Resolver" deprecated.

[kidunot89]

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What does this implement/fix? Explain your changes.

• Coupon_Connection_Resolver class deprecated and will be removed in release v0.11.x. The "PostObjectConnectionResolver" class and the hooks contained within are used in it's place. Any remaining unique code in-use has be relocated to the class-coupons.php.
• **CouponQueriesTest.php` has be refactored to use the WPGraphQLTestcase library.

Does this close any currently open issues?

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Any other comments?

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Where has this been tested?

• WooGraphQL Version: 0.8.1
• WPGraphQL Version: 1.3.8
• WordPress Version: 5.7.2
• WooCommerce Version: 5.3