build(deps-dev): bump phpunit/phpunit from 7.5.20 to 8.5.14

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 7.5.20 to 8.5.14.
- [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...8.5.14)

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

Author: dependabot[bot]

composer.json

@@ -27,7 +27,7 @@
         "php-http/mock-client": "^1.1",
         "phpspec/prophecy": "~1.0",
         "phpstan/phpstan": "^0.12",
-        "phpunit/phpunit": "^7"
+        "phpunit/phpunit": "^8"
     },
     "config": {
         "platform": {