Tweak Argon2 test config
Tweak Argon2 test config

Memory cost 8 seems to be lowest value accepted on my machine

```
Testing Symfony\Component\Security\Core\Tests\Encoder\Argon2iPasswordEncoderTest
E....                                                               5 / 5 (100%)

Time: 114 ms, Memory: 4.00MB

There was 1 error:

1) Symfony\Component\Security\Core\Tests\Encoder\Argon2iPasswordEncoderTest::testValidationWithConfig
password_hash(): Memory cost is outside of allowed memory range

/home/gadelat/PhpstormProjects/symfony/src/Symfony/Component/Security/Core/Encoder/Argon2iPasswordEncoder.php:105
/home/gadelat/PhpstormProjects/symfony/src/Symfony/Component/Security/Core/Encoder/Argon2iPasswordEncoder.php:67
/home/gadelat/PhpstormProjects/symfony/src/Symfony/Component/Security/Core/Tests/Encoder/Argon2iPasswordEncoderTest.php:34
```
symfony/Security