Update test environment to fix tests on legacy PHP 7.2 with PHPUnit 8.5

[clue]

This changeset updates the test environment to fix tests on legacy PHP 7.2 with PHPUnit 8.5. This is needed to run the tests on legacy platforms due to recent upstream changes:

• A recent security advisory (GHSA-vvj3-c3rp-c85p) affects most PHPUnit versions. It was patched in actively supported releases, but not in legacy PHPUnit versions, which we rely on for tests on legacy PHP < 7.3.
• Composer 2.9 now blocks installing packages with known advisories by default (Composer update: filter packages with security advisories from pool composer/composer#11956). Legacy PHP < 7.2 uses older Composer versions.
• While this project should not be affected by the advisory, both things combined mean that Composer would fail on legacy PHP 7.2 only ("Your requirements could not be resolved to an installable set of packages.")
• The suggested changes allow using PHPUnit 8.5 on PHP 7.2 until we remove legacy platforms in the future.

Builds on top of #104 and #127 and others

[Copilot]

Pull request overview

Updates dependency constraints and CI invocation to allow running the test suite on legacy PHP versions by permitting PHPUnit 8.5 and selecting the legacy PHPUnit config when needed.

Changes:

• Allow installing phpunit/phpunit ^8.5 in require-dev to support legacy PHP environments.
• Simplify the CI PHPUnit step by inlining the conditional selection of phpunit.xml.legacy for older PHP versions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
composer.json	Expands PHPUnit version constraints to include ^8.5 for legacy PHP compatibility.
.github/workflows/ci.yml	Consolidates PHPUnit run steps into one command with conditional legacy config selection.

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