Remove getImplementations compatibility layer

I'm marking this as a duplicate because it is resolved in this module's 8.x-4.x branch. It checks to see if the invokeAllWith method exists and then calls it. But, to support Drupal 8, they must leave the getImplementations function in place.

if (method_exists($this->moduleHandler, 'invokeAllWith')) {
      $this->moduleHandler->invokeAllWith('mail', function (callable $hook, string $module) use (&$list) {
        $list[$module] = $this->moduleHandler->getName($module);
      });
    }
    else {
      foreach ($this->moduleHandler->getImplementations('mail') as $module) {
        $list[$module] = $this->moduleHandler->getName($module);
      }
    }

Correct. But what we can do here is remove that and require 9.4.