Include grouped products for SKU and product identifier assessments

[hannaw93]

Context

Summary

This PR can be summarized in the following changelog entry:

• Fixes an unreleased bug where the SKU and Product identifier assessments did not work for Grouped products.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Activate Yoast SEO Free, Woocommerce and Yoast SEO for Woocommerce For devs: Run composer require yoast/wordpress-seo:dev-PC-522-add-SKU-identifiers-assessment-for-grouped-products@dev --dev before building in Free
• Create a new product
• Check that the SKU assessment shows orange and says: Your product is missing a SKU. Include this if you can, as it will help search engines to better understand your content.
• Hover over the links in the feedback strings. Confirm that the first link starts with <https://yoa.st/4lw> and the second one with <https://yoa.st/4lx>
• Change the product type to Grouped product in Product data
• Check that the assessment results do not change
• Add a SKU to the product
• Check that the SKU assessment says Your product has a SKU. Good job!
• Remove the SKU
• Check that the SKU assessment shows orange and says Your product is missing a SKU. Include this if you can, as it will help search engines to better understand your content.
• Add a title and save the product as a draft
• Check that the assessment still shows up and the result does not change
• Repeat the same steps for the Product identifiers assessment. (NOTE: for this assessment the feedback strings are expected to be <https://yoa.st/4ly> for the first link and <https://yoa.st/4lz> for the second link)

Relevant test scenarios

• [x] Changes should be tested with the browser console open
• [ ] Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• [ ] Changes should be tested on different editors (Block/Classic/Elementor/other)
• [ ] Changes should be tested on different browsers
• [ ] Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• [ ] QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• [ ] This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• [ ] This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• [ ] I have written documentation for this change.

Quality assurance

• [x] I have tested this code to the best of my abilities
• [ ] I have added unit tests to verify the code works as intended
• [ ] If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• [x] I have written this PR in accordance with my team's definition of done.

Fixes #https://yoast.atlassian.net/jira/software/c/projects/PC/boards/138?modal=detail&selectedIssue=PC-522

[manuelaugustin]

Note: let's not merge this PR for now since it's a change impacting Woo, but we don't know yet when we'll release the Woo add-on the next time.

[marinakoleva]

CRed. Adjusted testing instructions so that a title is added before the product is saved, because otherwise the grouped product switches back to a simple product and all SKU/PI entries are deleted. This is normal behaviour, apparently you can't save a grouped/ variable product in Woo without adding a title.

[hdvos]

CR: Small changes. All checks out. Good job!

[hdvos]

Acceptance test, happy to merge. All works as expected.