build(deps): bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/book.yml

@@ -25,7 +25,7 @@ jobs:
       contents: read
     steps:
       - name: Checkout the repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Install Python dependencies
         run: |

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
             path: "intro/mqtt/host-client"
     steps:
       - name: Checkout the repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Update ownership
         run: |

.github/workflows/docker_image.yml

@@ -35,7 +35,7 @@ jobs:
             path: "intro/mqtt/host-client"
     steps:
       - name: Checkout the repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Update ownership
         run: |
@@ -63,7 +63,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3