Update phpunit/phpunit requirement from ^6.5 to ^6.5 || ^8.0

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-8.0.md)
- [Commits](sebastianbergmann/phpunit@6.5.0...8.0.6)

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

composer.json

@@ -9,7 +9,7 @@
         "monolog/monolog": "^1.23"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.5",
+        "phpunit/phpunit": "^6.5 || ^8.0",
         "mockery/mockery": "^1.0",
         "overtrue/phplint": "^1.1"
     },