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

Author: renovate[bot]

.github/workflows/sync-issue-labels.yaml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Sync issue labels from docs repository
         run: gh label clone NordicSemiconductor/asset-tracker-cloud-docs --force

.github/workflows/test-and-release.yaml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-24.04
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: actions/setup-node@v6
         with:
           node-version: "22.x"

.github/workflows/update-repo-info.yaml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Sync repository description
         run: