Bump actions/checkout from 3 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...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/build_docker_images_for_ci.yml

@@ -35,7 +35,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Build the Docker image
       run: docker build . --file scripts/docker_files/docker_file_ci_ubuntu_22_04/DockerFile --tag kratosmultiphysics/kratos-image-ci-ubuntu-22-04
     - name: Docker Login
@@ -52,7 +52,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Build the Docker image
       run: docker build . --file scripts/docker_files/docker_file_ci_rockylinux8/DockerFile --tag kratosmultiphysics/kratos-image-ci-rocky8
     - name: Docker Login

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     outputs:
       files: ${{ steps.get_files.outputs.changed_files }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - id: get_files
         run: |
@@ -53,7 +53,7 @@ jobs:
       options: --user 1001
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     # - name: Installing dependencies
     # => must be added to the docker container to avoid reinstalling it in every CI run
@@ -254,7 +254,7 @@ jobs:
       KRATOS_CI_APPLICATIONS: ".github/workflows/ci_apps_windows.json"
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v6
       with:
         python-version: '3.8'
@@ -315,7 +315,7 @@ jobs:
       options: --user 1001
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
 
     # - name: Installing dependencies
     # => must be added to the docker container to avoid reinstalling it in every CI run
@@ -360,7 +360,7 @@ jobs:
       options: --user 1001
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: CI configuration
       shell: bash
@@ -470,7 +470,7 @@ jobs:
       KRATOS_CI_APPLICATIONS: "ONLY_CORE"
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v6
       with:
         python-version: '3.8'
@@ -560,7 +560,7 @@ jobs:
       options: --user 1001
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     # - name: Installing dependencies
     # => must be added to the docker container to avoid reinstalling it in every CI run

.github/workflows/jekyll-gh-pages.yml

@@ -28,13 +28,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout static page content
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           repository: KratosMultiphysics/Documentation
           ref: master
           path: statik_content
       - name: Checkout current branch
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           path: main
       - name: Configure page content

.github/workflows/nightly_build.yml

@@ -40,7 +40,7 @@ jobs:
       options: --user 1001
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: CI configuration
       shell: bash
@@ -231,7 +231,7 @@ jobs:
       KRATOS_BUILD_TYPE: Custom
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v6
       with:
         python-version: '3.8'
@@ -282,7 +282,7 @@ jobs:
       options: --user 1001
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v6
 
     - name: Installing dependencies
       shell: bash