Dropped support for PHP 7.2

[stephanvierkant]

See #341

This PR:

• Drops support for PHP 7.2
• Removes some useless docblock annotations (already as native type declarations)
• Adds return type on a public method of a final class
• Adds some small improvements now we can use PHP 7.3 features.

When this merged, I'll work on a PR that bumps the requirements to PHP 7.4.

I used Rector to update this code. This was my config:

use Rector\Core\Configuration\Option;
use Rector\Core\ValueObject\PhpVersion;
use Rector\Set\ValueObject\LevelSetList;
use Rector\Set\ValueObject\SetList;
use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;

return static function (ContainerConfigurator $containerConfigurator): void {
    // get parameters
    $parameters = $containerConfigurator->parameters();
    $parameters->set(Option::PHP_VERSION_FEATURES, PhpVersion::PHP_73);
    $parameters->set(Option::PATHS, [
        __DIR__ . '/src',
        __DIR__ . '/tests',
    ]);

    $parameters->set(Option::PHPSTAN_FOR_RECTOR_PATH, getcwd() . '/phpstan.neon');

    $containerConfigurator->import(SetList::DEAD_CODE);
    $containerConfigurator->import(LevelSetList::UP_TO_PHP_73);
};