[2.x] feat: sync abandoned extensions list from flarum/abandoned-extensions

[imorland]

Ports #4559 to 2.x.

Adds a weekly scheduled task that fetches the community-maintained flarum/abandoned-extensions list, filters it to installed packages, and surfaces the results in the admin panel.

What's included:

• AbandonedExtensionsFetcher — fetches, filters, diffs, and stores the list; optionally emails admins
• SyncAbandonedExtensionsCommand — CLI command with --notify flag, scheduled weekly
• POST /api/extensions/abandoned/sync — manual trigger for admins
• Basics page — "Check Now" button and a toggle to email admins when newly abandoned extensions are detected during the weekly check
• ExtensionManager — applies abandoned status from the stored map in extensionFromJson(), taking precedence over composer's own abandoned field

2.x adaptations:

• SendRawEmailJob → SendInformationalEmailJob (with named constructor args)
• Abandoned map applied in extensionFromJson() rather than inline in getExtensions()
• ExtensionServiceProvider uses constructor property promotion style
• BasicsPage uses the registry callback pattern (registerSetting() with a function) rather than overriding contentItems()
• Integration tests use Extend\ServiceProvider stub pattern; #[Test] attributes (PHPUnit 11)