[2.x] Upgrade to Laravel 13 before RC phase

[imorland]

Summary

Laravel 13 is scheduled for release on March 17, 2026. Flarum 2.x should be upgraded from Laravel 12 before we enter the RC phase, so we don't fall behind on a supported framework version.

Key points

• Laravel 13 drops PHP 8.2 support — Flarum's minimum PHP requirement will need to increase accordingly (8.3+). This also means updating CI/CD matrices, reusable workflows, and any composer.json platform constraints.
• No serious breaking changes are expected in L13, so the upgrade path should be relatively smooth.
• The official upgrade guide will be available at https://laravel.com/docs/13.x/upgrade from March 17th onwards.

Checklist

• Update flarum/core composer.json to require laravel/framework ^13.0
• Bump minimum PHP version from ^8.2 to ^8.3 across composer.json files
• Update reusable GitHub Actions workflows and CI matrices to drop PHP 8.2, add any new PHP versions
• Review the L13 upgrade guide for any breaking changes relevant to Flarum's usage
• Update CHANGELOG / release notes as appropriate
• Verify bundled extensions are compatible and update their composer.json constraints