ci: use Composer 2.2 for PHP 7.2 matrix entry

[tasuku43]

Summary

• Fix CI failure for PHP 7.2 by using Composer 2.2 instead of Composer 2

Problem

PHP 7.2 CI was failing with:

Root composer.json requires phpunit/phpunit ^6.0 || ^9.0 -> satisfiable by phpunit/phpunit[9.6.33, 9.6.34].
phpunit/phpunit[9.6.33, ..., 9.6.34] require php >=7.3 -> your php version (7.2.34) does not satisfy that requirement.

Composer 2.x (latest ~2.8) has different dependency resolution behavior than Composer 2.2 (LTS) and fails to resolve PHPUnit 6.x for PHP 7.2.

Solution

Use Composer 2.2 for PHP 7.2, aligning it with PHP 7.0 and 7.1 which already use Composer 2.2.

Test plan

• CI passes for all PHP versions including 7.2

🤖 Generated with Claude Code