Fix copilot-setup-steps: bump PHP to 8.4

doctrine/instantiator 2.1.0 in composer.lock requires PHP ^8.4.
The workflow was set to 8.3 which can't satisfy this constraint.

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

Author: dknauss

.github/workflows/copilot-setup-steps.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Set up PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: "8.3"
+          php-version: "8.4"
           tools: composer:v2
           coverage: none