ci: use Composer 2.2 for PHP 7.2 matrix entry

PHP 7.2 CI was failing because Composer 2.x (latest ~2.8) cannot resolve
PHPUnit 6.x dependency correctly, while Composer 2.2 (LTS) can.

This aligns PHP 7.2 with PHP 7.0 and 7.1 which already use Composer 2.2.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: tasuku43

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
             composer: 2.2
           - php: 7.2
             phpunit: 6
-            composer: 2
+            composer: 2.2
 
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1