Update phpunit/phpunit requirement from ^6.5 to ^7.5

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/7.5.20/ChangeLog-7.5.md)
- [Commits](sebastianbergmann/phpunit@6.5.0...7.5.20)

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

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

composer.json

@@ -82,7 +82,7 @@
     "lucatume/wp-browser": "^3.1",
     "lucatume/wp-snaphot-assertions": "^1.1",
     "phpcompatibility/phpcompatibility-wp": "*",
-    "phpunit/phpunit": "^6.5",
+    "phpunit/phpunit": "^7.5",
     "spatie/phpunit-snapshot-assertions": "^1.4.2",
     "the-events-calendar/coding-standards": "dev-master",
     "wp-cli/wp-cli": "2.*",