feat: support PHPUnit 12

[jdreesen]

Summary

• Allow phpunit/phpunit ^12.5 and extend the CI matrix (PHP >= 8.3 only, plus lowest-dependency runs; --fail-on-risky now applies to the newest PHPUnit).
• src/ needed no changes: every hook already carries an attribute besides its annotation (PHPUnit 12 ignores annotations), and the PHPUnit internals used by AttributeProvider are unchanged in 12.5.
• PHPUnit 12 validates the argument count while loading a data provider, i.e. before this library strips KernelConfiguration/PimcoreConfiguration objects from the provided data, and warns about it (failing runs with failOnWarning). Affected tests pass validateArgumentCount positionally (#[DataProvider('...', false)]), which PHPUnit 10/11 ignore, whereas the named argument would fail there with Unknown named parameter.
• README documents the warning for consumers (named argument for PHPUnit 12 only projects, positional/variadic variant for projects supporting several versions); CHANGELOG entry added under v0.15.1.
• Dropped the deprecated cacheDirectory attribute from <coverage> in phpunit.xml.dist.

Test plan

• Full suite locally with PHPUnit 12.5.38 / PHP 8.5.9 / --fail-on-risky: 109 tests, 0 failures, 0 warnings (remaining deprecations are PDO::MYSQL_ATTR_INIT_COMMAND from symfony/yaml on PHP 8.5)
• CI matrix incl. new PHPUnit 12 jobs and lowest-dependency runs (all green; the dama/doctrine-test-bundle lower bound did not need raising)
• PHPUnit 9/10/11 jobs still green with the positional false argument

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