cleanup last mention of ubuntu-20.04

Author: djmdjm

.github/workflows/c-cpp.yml

@@ -15,8 +15,8 @@ jobs:
       matrix:
         # First we test all OSes in the default configuration.
         target:
-          - ubuntu-20.04
           - ubuntu-22.04
+          - ubuntu-latest
           - ubuntu-22.04-arm
           - ubuntu-24.04-arm
           - macos-13