Update GitHub runner to supported version.

Author: chrisryan

.github/workflows/php.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         php-versions: ['7.3', '7.4', '8.0', '8.1']