Fix publish-box button layout on classic editor (WP 7.0+)

[enricobattocchi]

Context

• WP 7.0 made #major-publishing-actions a flex row, so our Copy / Rewrite links ended up squeezed onto the same row as Move to Trash and Update.

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where the Copy and Rewrite links broke the appearance of the Publish meta box in the Classic Editor on WordPress 7.0+.

Relevant technical choices:

• :has()-scoped flex-wrap: wrap on the core container plus flex-basis: 100% on our two buttons. No-op on WP 6.9 (parent isn't flex), no version check needed.

Test instructions

Test instructions for the acceptance test before the PR gets merged

• Open a published post in the classic editor on WP 7.0+ and confirm Copy / Rewrite are each on their own row above Trash + Update.
• Repeat on WP 6.9 if available: layout should be unchanged.

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

Classic editor only; block editor sidebar untouched.

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

• Classic editor Publish meta box only.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' 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
• I have added unittests to verify the code works as intended

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label and noted the work hours.

Fixes #

[enricobattocchi]

They are going probably to fix it on the WP side, so let's wait before moving that forward.