remove CI usage of ubuntu-20.04, just use ubuntu-latest

Author: folkertdev

.github/workflows/checks.yaml

@@ -186,7 +186,7 @@ jobs:
 
   fuzz:
     name: Smoke-test fuzzing targets
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         features: