Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/rust.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Build
       run: cargo build --verbose
     - name: Run tests