build(deps): 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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -73,7 +73,7 @@ jobs:
             python-version: '3.13.5'
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v5
@@ -137,7 +137,7 @@ jobs:
     runs-on: macos-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Build wheels
         uses: pypa/[email protected]
@@ -176,7 +176,7 @@ jobs:
   #   steps:
 
   #     - name: Checkout PyMAPDL
-  #       uses: actions/checkout@v4
+  #       uses: actions/checkout@v6
   #       with:
   #         repository: 'pyansys/pymapdl' #checking out main. Not release