ci: Update Ubuntu runner version (#503)

Author: jamesholland-uk

.github/workflows/ci.yml

@@ -90,7 +90,7 @@ jobs:
     name: Release
     if: github.event_name == 'push' && github.ref != 'refs/heads/develop'
     needs: [test, format]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@v2