Add PHP 8.3 to the GitHub Actions test matrix

Additionally, update actions/checkout from v2 to v4.

Author: stevegrunwell

.github/workflows/unit-tests.yml

@@ -8,10 +8,10 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-version: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2']
+        php-version: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3']
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2