Upgrade for Laravel 9.x (PHP 8.1, Flysystem 3.0)

[judgej]

Laravel 9.x uses PHP 8 and Flysystem 3. A two-stage upgrade should take us to PHP 8.1 first, then Flysystem 3.

• New Flysystem API.
• PHP 8.1 (dependencies and a few tweeks).
• Support new prefix method (League\Flysystem\PathPrefixer object).
• New Flysystem test suite (needs mocking of Azure classes, pretty involved; though maybe not if it actually runs against a live file share).
• Consistently apply all exceptions.
• Handle visibility.
• Update the docs, especially example results.