Add Laravel 13 and PHPUnit 12 compatibility

[feranmiemmanuel]

Summary

• Bump minimum PHP requirement from ^8.2 to ^8.3, matching Laravel 13's minimum
• Extend illuminate/support version constraint to include ^13.0
• Extend orchestra/testbench dev constraint to include ^11.0 (the testbench branch targeting Laravel 13)
• Extend pestphp/pest dev constraint to include ^4.0 (required for PHPUnit 12 compatibility)
• Update phpunit.xml schema to PHPUnit 12 and remove attributes that were deprecated in PHPUnit 11 and removed in PHPUnit 12 (backupGlobals, backupStaticProperties, processIsolation, stopOnFailure)
• Update README.md to reflect the new PHP 8.3+ minimum

Test plan

• composer install resolves without conflicts
• ./vendor/bin/pest passes — 22 tests pass, 0 failures (verified locally)
• Confirm the package installs cleanly in a fresh Laravel 13 project (composer require cloudinary-labs/cloudinary-laravel)
• Smoke-test the cloudinary storage disk driver registers and can upload/read/delete a file against a real Cloudinary account

[vercel]

@feranmiemmanuel is attempting to deploy a commit to the Cloudinary DevX Team on Vercel.

A member of the Team first needs to authorize it.