Update actions/checkout action to v6

Author: simplificator-renovate[bot]

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
 
     steps:
       - name: Check out the codebase.
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           path: "simplificator.linux-accounts"