Update PHPUnit to 9.5.5

[morozov]

Q	A
Type	improvement
BC Break	no

PHPUnit 9.5.5 improves compatibility with PHP 8.1. Specifically, in sebastianbergmann/phpunit@3e3aecd by addressing the Serializable deprecation.

Currently, if run on PHP 8.1, the test suite produces the following errors:

$ phpunit
PHP Deprecated:  The Serializable interface is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in vendor/phpunit/phpunit/src/Runner/DefaultTestResultCache.php on line 34

Deprecated: The Serializable interface is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in vendor/phpunit/phpunit/src/Runner/DefaultTestResultCache.php on line 34
PHPUnit 9.5.0 by Sebastian Bergmann and contributors.

Note, I don't believe DBAL 2.x will support PHP 8.1 but as long this release is supported, we should update dependencies starting the oldest supported release.