Update ci.yml

Author: dktapps

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   build:
     name: PHP ${{ matrix.php }} (ZTS) Valgrind ${{ matrix.valgrind }}
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     if: "!contains(github.event.head_commit.message, '[ci skip]')"
 
     strategy: