Update composer/composer requirement from ^1.8 to ^2.0

Updates the requirements on [composer/composer](https://github.com/composer/composer) to permit the latest version.
- [Release notes](https://github.com/composer/composer/releases)
- [Changelog](https://github.com/composer/composer/blob/master/CHANGELOG.md)
- [Commits](composer/composer@1.8.0...2.0.8)

Signed-off-by: dependabot[bot] <support@github.com>

composer.json

@@ -11,7 +11,7 @@
         "phpstan/phpstan": ">=0.11.6"
     },
     "require-dev": {
-        "composer/composer": "^1.8",
+        "composer/composer": "^2.0",
         "phing/phing": "^2.16.3",
         "php-parallel-lint/php-parallel-lint": "^1.2.0",
         "phpstan/phpstan-strict-rules": "^0.11 || ^0.12"