ci: 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] <[email protected]>

Author: dependabot[bot]

.github/workflows/main.yml

@@ -32,7 +32,7 @@ jobs:
           # needed to avoid MPI warning
           echo "OMPI_MCA_btl=^openib" >> $GITHUB_ENV
     #we need to checkout to access .ci/install.libtorch
-    # - uses: actions/checkout@v5
+    # - uses: actions/checkout@v6
     #   with:
     #     persist-credentials: false
 
@@ -73,7 +73,7 @@ jobs:
         python-version: ["3.8", "3.9", "3.10", "3.11", "3.12","3.13"]
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         persist-credentials: false
     - name: Install plumed dependencies

.github/workflows/release.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Python
       uses: actions/setup-python@v6
       with: