Update phpunit/phpunit requirement from ^7.5.20 to ^8.5.38

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/8.5.38/ChangeLog-8.5.md)
- [Commits](sebastianbergmann/phpunit@7.5.20...8.5.38)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
...

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

composer.json

@@ -43,7 +43,7 @@
   "require-dev": {
     "dealerdirect/phpcodesniffer-composer-installer": "^0.7.0",
     "php-stubs/wp-cli-stubs": "^2.4",
-    "phpunit/phpunit": "^7.5.20",
+    "phpunit/phpunit": "^8.5.38",
     "szepeviktor/phpstan-wordpress": "^1.1",
     "wearerequired/coding-standards": "^3.0",
     "wp-cli/extension-command": "^2.0",