Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
   build:
     runs-on: ubuntu-24.04
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         submodules: 'true'
     - name: install build dependencies

.github/workflows/verify.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest  # container actions require GNU/Linux
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         persist-credentials: false
         submodules: 'true'