Update phpunit/phpunit requirement from ^7.5 to ^8.2 (#21)

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/master/ChangeLog-8.2.md)
- [Commits](sebastianbergmann/phpunit@7.5.0...8.2.1)

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

Co-authored-by: null <dependabot-preview[bot]@users.noreply.github.com>

composer.json

@@ -23,7 +23,7 @@
     "require-dev": {
         "graham-campbell/testbench": "^5.0",
         "mockery/mockery": "^1.0",
-        "phpunit/phpunit": "^7.5"
+        "phpunit/phpunit": "^8.2"
     },
     "autoload": {
         "psr-4": {