Your requirements could not be resolved to an installable set of packages.
Problem 1
- Root composer.json requires phpunit/phpunit 9.6 -> satisfiable by phpunit/phpunit[9.6.0].
- phpunit/phpunit 9.6.0 requires sebastian/diff ^4.0.3 -> found sebastian/diff[4.0.3, ..., 4.0.x-dev] but it conflicts with your root composer.json require (4.0).
Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.