Update phpunit/phpunit requirement from ^6.5 to ^6.5 || ^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/master/ChangeLog-7.5.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/7.5.2)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support

composer.json

@@ -26,7 +26,7 @@
         "psr/http-server-handler": "1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.5",
+        "phpunit/phpunit": "^6.5 || ^7.0",
         "zendframework/zend-diactoros": "^1.7 || ^2.0"
     },
     "suggest": {