.github: Update actions/checkout to v4

The current version of https://github.com/actions/checkout is v4,
so let's use it.

Author: neuschaefer

.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 }}