Update PHPUnit 10

[jorgsowa]

Updates PHPUnit version to 10.

• migrated PHPUnit configuration to v10 and left old configuration as phpunit.xml.9.5.dist
• modified GitHub actions to run proper PHPUnit configuration depending on the PHP version
• declared all data providers as static,
• added PHPUnit version ^10.0 in Composer,
• added PHPUnit 10 cache dir to .gitignore,

[nikic]

migrated PHPUnit configuration to v10 and left old configuration as phpunit.xml.9.5.dist

Is this necessary? Doesn't PHPUnit automatically migrate the old config?

[jorgsowa]

Is this necessary? Doesn't PHPUnit automatically migrate the old config?

XML of PHPUnit v10 is not compatible with XML of PHPUnit v9. Joining it with strictness of warnings it's not possible to use the same PHPUnit configuration file for both versions of 9 and 10.

Example of failing test with PHPUnit 10 but with elements of XML from PHPUnit 9: https://github.com/nikic/PHP-Parser/actions/runs/9086446408/job/24979454495?pr=998

The idea I came up with is to have simply two configuration files for each PHPUnit version. The other solution would be to upgrade completely to PHPUnit 10, but it would require bumping min. PHP version to 8.1 which is not desired for now.

[nikic]

I think overall I'm not clear on what the benefit of using PHPUnit 10 on some versions and PHPUnit 9 on some is. This just seems to increase complexity, as we wouldn't be able to use any new functionality from PHPUnit 10 anyway. If it "just worked", that would be one thing, but apparently it doesn't...

(I think merging just the data provide changes would make sense though, in the interest of forward-compatibility.)