VariationAttribute field "id" updated.

[kidunot89]

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What does this implement/fix? Explain your changes.

• ProductVariation field id has been changed to a cacheable global ID created from the parent product or product variation id, the attribute name, the attribute value. This ID is not design for server-side retrieval functionality, although, it's possible.
• The old id field is accessible from attributeId. The value is the WP_Term ID, if the product attribute is a WP taxonomy, otherwise is 0.

Does this close any currently open issues?

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Any other comments?

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Where has this been tested?

Operating System: Ubuntu 18.04

WordPress Version: 5.2