Bump phpunit/phpunit from 8.5.2 to 9.1.1

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 8.5.2 to 9.1.1.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.5.md)
- [Commits](sebastianbergmann/phpunit@8.5.2...9.1.1)

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

composer.json

@@ -12,7 +12,7 @@
 
     },
     "require-dev": {
-      "phpunit/phpunit": "~8.5",
+      "phpunit/phpunit": "~9.1",
       "wp-cli/wp-cli": "~2.4",
       "wp-coding-standards/wpcs": "~2.2",
       "dealerdirect/phpcodesniffer-composer-installer": "^0.6",