Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
     - name: checkout
-      uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
     - name: lint
       run: cargo fmt -- --check
     - name: build