CI: Run on Ubuntu 20.04

The 18.04 runners are not available anymore as per
actions/runner-images#6002 so use
instead the lowest available version at the moment. While at it, update
to the latest version of the upload-artifact action, as v1 is
deprecated.

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ on: [push]
 
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -55,7 +55,7 @@ jobs:
         run: |
           DESTDIR="$(pwd)/_work/files" ninja -C _work/build install
       - name: Archive Artifacts
-        uses: actions/upload-artifact@v1
+        uses: actions/upload-artifact@v4
         with:
           name: build
           path: _work