build(deps-dev): update phpunit/phpunit requirement || ^7.0

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/7.5.18/ChangeLog-7.5.md)
- [Commits](sebastianbergmann/phpunit@6.0.0...7.5.18)

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

composer.json

@@ -31,7 +31,7 @@
         "twig/twig":  "^2.4.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0"
+        "phpunit/phpunit": "^6.0 || ^7.0"
     },
     "bin": ["bin/phint"],
     "config": {