Bump actions/checkout from 5 to 6 (#979)

Author: dependabot[bot]

.github/workflows/humble-docker-build.yml

@@ -26,7 +26,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         ref: humble
     - name: Build the Docker image

.github/workflows/jazzy-docker-build.yml

@@ -26,7 +26,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         ref: jazzy
     - name: Build the Docker image

.github/workflows/rolling-docker-build.yaml

@@ -32,7 +32,7 @@ jobs:
       matrix:
         ROS_DISTRO: [rolling, kilted]
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         ref: master
     - name: Build the Docker image