fix(deps): require PHP >=8.3 to match typed class constants

ContributionStore uses a typed class constant (PHP 8.3+), but composer.json
still allowed 8.2. PHPStan 2.3 reads the constraint and fails on it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

composer.json

@@ -4,7 +4,7 @@
     "type": "project",
     "license": "MIT",
     "require": {
-        "php": ">=8.2",
+        "php": ">=8.3",
         "monolog/monolog": "^3.10",
         "symfony/cache": "7.4.*",
         "symfony/console": "7.4.*",