Update CI workflow

Author: LukasKalbertodt

.github/workflows/ci.yml

@@ -14,14 +14,14 @@ jobs:
     name: Check basic style
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: LukasKalbertodt/[email protected]
 
   check:
     name: 'Build & test'
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     # We test in release mode as two tests would take a long time otherwise.
     - name: Build