Drop PHP 7.4 from CI matrix, add 8.3 and 8.4

Foundry uses `mixed` type hints (PHP 8.0+) in QueryResults, so 7.4 is
not supported. Added 8.3 and 8.4 to cover current PHP versions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: joehoyle

.github/workflows/phpunit.yml

@@ -18,7 +18,7 @@ jobs:
 
     strategy:
       matrix:
-        php: ['7.4', '8.0', '8.1', '8.2']
+        php: ['8.0', '8.1', '8.2', '8.3', '8.4']
 
     steps:
       - uses: actions/checkout@v4