Updated GitHub PHPUnit workflow

Added PHP 8.3 testing coverage.
Updated action versions

.github/workflows/phpunit.yml

@@ -7,13 +7,13 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: [7.2, 7.3, 7.4, 8.0, 8.1, 8.2]
+        php: [7.2, 7.3, 7.4, 8.0, 8.1, 8.2, 8.3]
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Cache dependencies
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ~/.composer/cache/files
           key: dependencies-php-${{ matrix.php }}-composer-${{ hashFiles('composer.json') }}