Fix introduction gradient.

[vraja-pro]

Context

• We want to use the introduction gradient class in a contained way also in the ai title and description introduction modal.

Summary

This PR can be summarized in the following changelog entry:

• Fix gradient background for AI title and description generator introduction modal.

Relevant technical choices:

• I removed the yst-introduction-modal-panel class because we want to use the gradient inside elements in the modal.
• We want more flexibility when using the gradient.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Either clean the user meta _yoast_wpseo_introductions or create a new site.
• Go to one of Yoast admin pages and check the introduction modal has a gradient of 50% only on the upper part of the modal.
• Edit a post and click on the Use AI button and check the upsell modal for the AI title and description generator has the same gradient.
• Click on the AI buttons in the SEO Analysis assessments and check the upsell modal has the same gradient.

Relevant test scenarios

• 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 (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

• 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. 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

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• 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.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

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

Fixes 23.2-Free-AI: Upsell/Intro modal design issue

[pls78]

CR && Acc: ✅