Github CI update orphaned OS

Github Actions does not have Ubuntu 16.04 boxes any longer.

Going to Ubuntu 18.04 as the actions came to an halt.

Author: ktomk

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        machine: ['ubuntu-16.04']
+        machine: ['ubuntu-18.04']
         php-version: ['7.0', '7.1', '7.2', '7.3', '7.4']
         run-job: ['env.1', 'env.2']
         experimental: [false]