.github: Update actions/checkout to v4

https://github.com/actions/upload-artifact warns:

> actions/upload-artifact@v3 is scheduled for deprecation on November
> 30, 2024. Learn more. Similarly, v1/v2 are scheduled for deprecation
> on June 30, 2024.

.github/workflows/c-cpp.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-24.04
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: install cython
       run: |
         sudo apt-get update && sudo apt-get install -y ${{ matrix.cython }}