Upgrade to Ubuntu 20 for all CI (#20)

Author: iamcal

.github/workflows/php.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   build:
 
-    runs-on: ${{ matrix.operating-system }}
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         operating-system: ['ubuntu-20.04']
@@ -18,35 +18,25 @@ jobs:
         coverage: [false]
         include:
         - php-versions: '5.3'
-          operating-system: 'ubuntu-16.04'
           phpunit-versions: '4.8.36'
         - php-versions: '5.4'
-          operating-system: 'ubuntu-16.04'
           phpunit-versions: '4.8.36'
         - php-versions: '5.5'
-          operating-system: 'ubuntu-16.04'
           phpunit-versions: '4.8.36'
         - php-versions: '5.6'
-          operating-system: 'ubuntu-16.04'
           phpunit-versions: '4.8.36'
         - php-versions: '7.0'
-          operating-system: 'ubuntu-16.04'
           phpunit-versions: '4.8.36'
         - php-versions: '7.1'
-          operating-system: 'ubuntu-18.04'
           phpunit-versions: '4.8.36'
         - php-versions: '7.2'
-          operating-system: 'ubuntu-18.04'
           phpunit-versions: '4.8.36'
         - php-versions: '7.3'
-          operating-system: 'ubuntu-18.04'
           phpunit-versions: '7.5.20'
         - php-versions: '7.4'
-          operating-system: 'ubuntu-20.04'
           phpunit-versions: '7.5.20'
           coverage: true
         - php-versions: '8.1'
-          operating-system: 'ubuntu-20.04'
           phpunit-versions: '9.5.4'
 
     steps: