composer(deps-dev): update orchestra/testbench requirement (#423)

Updates the requirements on [orchestra/testbench](https://github.com/orchestral/testbench) to permit the latest version.
- [Release notes](https://github.com/orchestral/testbench/releases)
- [Changelog](https://github.com/orchestral/testbench/blob/v7.20.0/CHANGELOG-7.x.md)
- [Commits](orchestral/testbench@v7.0.0...v7.20.0)

---
updated-dependencies:
- dependency-name: orchestra/testbench
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

composer.json

@@ -38,7 +38,7 @@
         "dg/bypass-finals": "^1.3",
         "mikey179/vfsstream": "^1.6",
         "mockery/mockery": "^1.5",
-        "orchestra/testbench": "^7.0",
+        "orchestra/testbench": "^7.20",
         "phpunit/phpunit": "^9.5.28"
     },
     "minimum-stability": "dev",