[2.x] Update to opis/closure v4 and fix PHPStan

[dsevillamartin]

Changes proposed in this pull request:

• Replace the closure package, which itself provided more modern support for opis/closure back in the day, with the original package that supports PHP 8.0-8.5. The old one had not been updated in 3+ years.

  PHP Deprecated:  Opis\Closure\unserialize(): Implicitly marking parameter $options as nullable is deprecated, the explicit nullable type must be used instead in [vendor]/charescape\serialize-closure\functions.php
  

  There was a package rewrite with v4, but this only affects the data unserializing, and there's a v3 compatibility mode I enabled in case a queued job was created before the upgrade -- though not sure if we should really support this use case anyways. Unsure of what the harm on enabling the compat mode is.

• Update PHPStan config so that it works again after major upgrade in flarum/phpstan package

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).