Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/secretlint-only-on-changed-files.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - uses: technote-space/get-diff-action@v6
       - name: setup Node.js
         uses: actions/setup-node@v4

.github/workflows/secretlint-via-docker-only-on-changed-files.yml

@@ -9,7 +9,7 @@ jobs:
     name: "Secretlint via Docker only on changed files"
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: technote-space/get-diff-action@v6
       - name: Run secretlint command on docker
         uses: addnab/docker-run-action@v3

.github/workflows/secretlint.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with: