4580 fix ai modal when using astra plugins

[mykola]

Context

• Fix ai modal when using astra plugins.

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where Yoast modals would not be visible when using Astra plugins.

Relevant technical choices:

• Astra plugins set z-index 10000 for WpModal we use for Search Appearance. The same value used for headlessui modals used for AI. So solution is to increase z-index for headlessui modals

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Install and activate Yoast SEO (free and premium).
• Install plugin Starter Templates or Spectra.
• Edit a post to add a keyword.
• Update post.
• Open the 'Search Appearance' in the sidebar.
• Click the 'Use AI' button.
• AI pop-up should appear correctly.

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:

• Should not impact other places.

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 #4580

[Jordi-PV]

CR + AT 👍