Run ci on ubuntu-latest

Author: AlistairB

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   ci:
     timeout-minutes: 30
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     name: ${{ matrix.ghc }}-${{ matrix.deb }}
     strategy:
       fail-fast: false