Update pixelmatch requirement in /tests/ui-regression
Update pixelmatch requirement in /tests/ui-regression

Updates the requirements on [pixelmatch](https://github.com/mapbox/pixelmatch) to permit the latest version.
- [Release notes](https://github.com/mapbox/pixelmatch/releases)
- [Commits](https://github.com/mapbox/pixelmatch/compare/v4.0.2...v5.0.2)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,nextcloud/server,nextcloud/server,nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server