Update phpunit/phpunit requirement from ^9.5.21 to ^9.6.19

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/9.6.19/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.5.21...9.6.19)

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

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

composer.json

@@ -31,7 +31,7 @@
     "require-dev": {
         "orchestra/testbench": "^7.14",
         "orchestra/testbench-core": "^7.14",
-        "phpunit/phpunit": "^9.5.21",
+        "phpunit/phpunit": "^9.6.19",
         "vimeo/psalm": "4.24.0"
     },
     "config": {