[2.x] revert(admin): remove collapsible extension categories, restore 3-section sidebar

[imorland]

Summary

Reverts the collapsible extension category sidebar introduced in #4392, replacing it with the previous 3-section flat list (Features, Themes, Languages).

The collapsible groups caused more problems than they solved:

• Everything collapsed by default — you open the admin panel and can see zero extensions
• Count badges (orange 27) read like unread notification counts, creating false urgency
• Single-item categories (Infrastructure: 1, Analytics: 1, Themes: 1) added a click with no payoff
• Extensions with specific categories (moderation, discussion, etc.) would have been invisible until manually expanded

What's kept from #4392

The rest of that PR is preserved:

• Extension health widget on the dashboard (abandoned packages, suggestions, disabled extensions)
• Abandoned package ! badges on individual extension sidebar items
• ExtensionLinkButton tooltip showing title and version
• ExtensionManager::getInstalledPackageNames() and AdminPayload abandoned override logic

Changes

• AdminNav.js: removed collapsed state, isCollapsed/toggleCollapsed/categoryIcon methods, restored plain <h4> category headers
• AdminApplication.tsx: reverted extensionCategories to { feature: 30, theme: 20, language: 10 }
• AdminNav.less: reverted .ExtensionListTitle to simple muted uppercase heading, removed 5 new CSS classes

Extensions with specific categories (moderation, discussion, formatting, etc.) fall back to the Features section via the existing getCategorizedExtensions fallback behaviour.