Allow Symfony 6 and resolve depreciations

[GromNaN]

First, thank you for maintaining this library. I work on a project that uses it and that needs to be upgraded to Symfony 6.0.

I had to make a lot of updates in order to have tests passing. Please tell me if you prefers separated pull requests, or anything that I should change.

• Remove hirak/prestissimo, not necessary since Composer 2 supports parallel downloads.
• Switch to symfony/simple-phpunit to get polyfills for PHPUnit versions.
• Require PHP 7.1+ (void return type)
• Allow Symfony 6.0 and add return types.
• Replace Mockery with PHPUnit Mock in Cocur\Slugify\Tests\Bridge\Symfony\CocurSlugifyExtensionTest, to solve an error ParseError: syntax error, unexpected token "static", expecting identifier

[GromNaN]

Good catch. There is issue with scrutinizer. Not sure if it is correctly configured.