Bump phpunit/phpunit from 7.5.20 to 9.5.6

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 7.5.20 to 9.5.6.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-9.5.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/compare/7.5.20...9.5.6)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

composer.json

@@ -30,7 +30,7 @@
 		"spatie/color": "^1.1"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^7",
+		"phpunit/phpunit": "^9",
 		"pressbooks/coding-standards": "^1.0",
 		"wpreadme2markdown/wp2md": "^3.0"
 	},