CI: Update image to Ubuntu 18.04

Bump the version of the Ubuntu image used for CI builds to 18.04, which
is the previous-to-last LTS version currently available, and the oldest
system where building libwpe is currently supported.

Author: aperezdc

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ on: [push]
 
 jobs:
   build:
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2