Bump ci ubuntu version to fix setup-php build

see shivammathur/setup-php#452

Author: Jeremiah VALERIE

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ env:
 
 jobs:
   build:
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: