Enables AI suggestions for taxonomies

[mhkuu]

Context

• We want to enable the AI generator functionality for taxonomies as well. This PR enables the "Use AI" button for taxonomies in Free (with an upsell for the actual AI generator functionality in Premium). It also changes some copy so that's it's clear that our functionality is more generic.

Summary

This PR can be summarized in the following changelog entry:

• Enables the AI generator functionality for taxonomies.

Relevant technical choices:

• I've added an integration test for PHP, I did not add tests for the (small) JS changes.

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, not Premium.
• In the Settings, confirm that the AI generator functionality is greyed out.
• Confirm that the card copy reads: Use AI to write SEO titles and meta descriptions for your content. It speeds up your work and does some of the thinking for you!.
• Create a new taxonomy (e.g. a category, tag, or a custom taxonomy).
• Confirm that a 'Use AI' button is shown when editing the taxonomy that you just created.
• Confirm that clicking on the 'Use AI' button renders a modal with an upsell.
• Confirm that the microcopy reads Let AI do some of the thinking for you and help you save time. Get high-quality suggestions for titles and meta descriptions to make your content rank high and look good on social media. Learn more ->.

Confirm that 'Use AI' is not shown for Woo taxonomies.

• Activate WooCommerce.
• Create a new Woo taxonomy (a Woo product category or Woo product tag).
• Confirm that the 'Use AI' button is not shown when editing the taxonomy that you just created.

Relevant test scenarios

• [x] Changes should be tested with the browser console open
• [x] Changes should be tested on different taxonomies/custom taxonomies
• [ ] Changes should be tested on different editors (Default Block/Gutenberg/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

• [x] 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:

• The AI generator functionality for posts / products is unchanged, save for some copy changes.

UI changes

• [x] 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. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• [x] I have tested this code to the best of my abilities.
• [x] During testing, I had activated all plugins that Yoast SEO provides integrations for.
• [x] 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.
• [x] I have checked that the base branch is correctly set.

Innovation

• [ ] No innovation project is applicable for this PR.
• [x] This PR falls under an innovation project. I have attached the innovation label.
• [x] I have added my hours to the WBSO document.

Fixes https://github.com/Yoast/lingo-other-tasks/issues/339