Admin: Standardize padding in menu item source accordions

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/63438

This PR addresses CSS padding inconsistencies within the Customizer accordion sections. Specifically, it standardizes the padding for lists of available items (Pages, Posts, etc.) and the "Custom Links" section.

Updates the padding property for the affected selectors in both wp-admin/css/nav-menus.css and wp-admin/css/customize-nav-menus.css to a uniform 15px on all sides. This enhances visual consistency and improves the user experience in these core menu management interfaces.

Screencast

Before

Screen.Recording.2025-05-13.at.13.07.33.mov

After

Screen.Recording.2025-05-13.at.13.07.01.mov

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