PHP 8 compatability

[nie7321]

Due to the php ~7.1 requirement in composer.json, this package is not installable on PHP 8.

However, the tests pass on 8, with a couple easy-to-fix warnings:

vagrant@homestead:~/code/SolrQueryComponent$ php --version
PHP 8.0.1 (cli) (built: Jan 13 2021 08:22:35) ( NTS )
Copyright (c) The PHP Group
Zend Engine v4.0.1, Copyright (c) Zend Technologies
    with Zend OPcache v8.0.1, Copyright (c), by Zend Technologies

vagrant@homestead:~/code/SolrQueryComponent$ phpunit
PHP Warning:  Private methods cannot be final as they are never overridden by other classes in /home/vagrant/code/SolrQueryComponent/vendor/phpunit/phpunit/src/Util/Configuration.php on line 176

Warning: Private methods cannot be final as they are never overridden by other classes in /home/vagrant/code/SolrQueryComponent/vendor/phpunit/phpunit/src/Util/Configuration.php on line 176
PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.0.1
Configuration: /home/vagrant/code/SolrQueryComponent/phpunit.xml.dist
Error:         No code coverage driver is available

...............................................................  63 / 104 ( 60%)
.........................................                       104 / 104 (100%)

Time: 276 ms, Memory: 8.00MB

OK (104 tests, 301 assertions)

I'm happy to submit the PRs to fix this (here & in the dependent packages InterNations owns) if you'd like!