composer(deps-dev): bump phpunit/phpunit from 7.5.20 to 8.5.8

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 7.5.20 to 8.5.8.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.5.md)
- [Commits](sebastianbergmann/phpunit@7.5.20...8.5.8)

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

composer.json

@@ -33,7 +33,7 @@
     "phpstan/phpstan": "~0.12.40",
     "phpstan/phpstan-deprecation-rules": "~0.12.5",
     "phpstan/phpstan-strict-rules": "~0.12.4",
-    "phpunit/phpunit": "^7.5.20",
+    "phpunit/phpunit": "^8.5.8",
     "psalm/plugin-phpunit": "~0.11.0",
     "vimeo/psalm": "^3.14.2",
     "zendframework/zend-file": "^2.8.3"