Change Github Workflow to use ubuntu-latest.

Author: chrisryan

.github/workflows/php.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         php-versions: ['7.2', '7.3', '7.4', '8.0', '8.1', '8.2']