Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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/clang.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         submodules: true
 

.github/workflows/doxyfile.yml

@@ -32,7 +32,7 @@ jobs:
     steps:
 
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           # Get the libprimis-header submodules.
           submodules: true

.github/workflows/makefile.yml

@@ -12,7 +12,7 @@ jobs:
         cpp_version: [20]
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         submodules: true
 

.github/workflows/msbuild.yml

@@ -17,7 +17,7 @@ jobs:
 
     steps:
     - name: Clone repository and submodules
-      uses: actions/checkout@v4
+      uses: actions/checkout@v6
       with:
         submodules: true
 

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
         cpp_version: [20,23]
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         submodules: true