ubuntu version update

Author: adityapatadia

.github/workflows/php-testing.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        operating-system: [ubuntu-18.04]
+        operating-system: [ubuntu-20.04]
         php-versions: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1']
     name: PHP ${{ matrix.php-versions }} Test on ${{ matrix.operating-system }}
     steps: