GH Actions: update for the release of PHP 8.3

... which is expected later today.

Author: jrfnl

.github/workflows/test.yml

@@ -85,7 +85,7 @@ jobs:
   test:
     name: Run tests on PHP ${{ matrix.php }}
     runs-on: ubuntu-latest
-    continue-on-error: ${{ matrix.php == '8.3' }}
+    continue-on-error: ${{ matrix.php == '8.4' }}
     needs:
       - bundle
 
@@ -105,6 +105,7 @@ jobs:
           - '8.1'
           - '8.2'
           - '8.3'
+          - '8.4'
 
     steps:
       - name: Checkout code