Update plugins/builtin example to use php83 instead of php82

The php82 version (8.2.29) in nixpkgs-unstable has a flaky test in the
php-dom extension (gh10234.phpt) that causes intermittent CI failures.

Moving to php83 (which successfully built as version 8.3.3 on Oct 23)
provides a conservative upgrade path while avoiding the flaky test.

This is preferable to modifying the PHP plugin flake to skip all tests,
as it allows us to keep upstream testing while avoiding the specific
problematic version.

Fixes the Oct 24 CI failure in the plugins_builtin_run_test.test.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: savil

examples/plugins/builtin/devbox.json

@@ -11,7 +11,7 @@
     }
   },
   "include": [
-    // Installs the php plugin using php82 as trigger package
-    "plugin:php82"
+    // Installs the php plugin using php83 as trigger package
+    "plugin:php83"
   ]
 }