Bump to PHP 8.0+

PHP 7.4 was end of life November 2022, it's time to bump to PHP 8 standard 🎉

- updated starting code to PHP 8 standard
- upgraded tooling (code quality, static analysis) to the latest versions
- tested with PHP 8.0, 8.1 and 8.2
   - the fixture is working
   - the example test is failing, as expected
   - code quality is as expected
   - static analysis is ok

php/fixtures/texttest_fixture.php

@@ -25,7 +25,7 @@ $items = [
 $app = new GildedRose($items);
 
 $days = 2;
-if (count($argv) > 1) {
+if ((is_countable($argv) ? count($argv) : 0) > 1) {
     $days = (int) $argv[1];
 }