Remove PHP 7.3 from workflow as it's EOL

Author: patrikjuvonen

.github/workflows/php.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-versions: ['7.3', '7.4', '8.0']
+        php-versions: ['7.4', '8.0']
     name: Build PHP version ${{ matrix.php-versions }}
     steps:
     - uses: actions/checkout@v2