Update coding style for upcoming PHP-CS-Fixer changes

Once we bump minimum PHP version, we will get newer PHP-CS-Fixer,
which will try to apply this cleanups.

Also manually tweak anonymous functions so that they are cleanly formatted
once we switch to `fn` syntax.

.php-cs-fixer.php

@@ -26,6 +26,10 @@ return (new PhpCsFixer\Config())
         'strict_comparison' => true,
         'strict_param' => true,
         'concat_space' => ['spacing' => 'one'],
+        // Pulled in by @Symfony:risky but we still support PHP 7.4
+        'modernize_strpos' => false,
+        // Pulled in by @Symfony, we cannot add property types until we bump PHP to ≥ 7.4
+        'no_null_property_initialization' => false,
     ])
     ->setFinder($finder)
 ;