Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/cmake.yml

@@ -18,7 +18,7 @@ jobs:
     - name: Install dependencies
       run: sudo apt install libboost-dev
 
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         submodules: "true"