PHPUnit 9.3 released a new schema for the phpunit.xml configuration file. I had to migrate the file to the new format in order to avoid the warning. PHPUnit Versions older than 9.3 have to use the phpunit.xml.legacy configuration file because the new format is unknown for them.
For further details concerning this pull request look into graphp/graphviz #46.

It's also possible to run this code with PHP 8 (without extension) 🎉

$ docker run -it --rm -v `pwd`:/data --workdir=/data php:8.0.0 php vendor/bin/phpunit
PHPUnit 9.5.1 by Sebastian Bergmann and contributors.
..SSSSSSSSSSSSSSSSSSSSSSSSSSSS.SSSS..SSSSSSSSSSSSSSSSSSSSSSSSSS  63 / 140 ( 45%)
SS.SSSS..SSSSSSSSSSSSSSSSSSSSSSSSSSSS.SSSS..................... 126 / 140 ( 90%)
...........SS.                                                  140 / 140 (100%)
Time: 00:05.394, Memory: 450.01 MB
OK, but incomplete, skipped, or risky tests!
Tests: 140, Assertions: 88, Skipped: 98.