chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/test.yml

@@ -19,7 +19,7 @@ jobs:
     name: Unit tests (bats)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive
 
@@ -35,7 +35,7 @@ jobs:
     name: Shellcheck
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Run shellcheck
         run: shellcheck --severity=error src/crabcode