Whats New: Make it on the center of the screen

[arthur791004]

Related to #88324

Proposed Changes

• Move the What's New modal to the center of the screen

Before	After

Testing Instructions

• Go to your site with the nav redesign
• Make sure the What's New modal is on the center of the screen. If the modal is not shown, you can open the Help Center and scroll down to click "What's new"

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• https://wpcalypso.wordpress.com/devdocs/docs/testing/index.md for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-100341

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-100341&env=jetpack

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• blaze-dashboard
• command-palette-wp-admin
• editing-toolkit
• odyssey-stats
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug feat/center-whats-new-modal on your sandbox.

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[roo2]

With making it centered, we'd better make the background clickable again like p1709821089976579/1709762884.683639-slack-C06DN6QQVAQ

[roo2]

openned #88438