ci: bump actions/upload-artifact from 5 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  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

@@ -58,7 +58,7 @@ jobs:
         tar cf plumed-master.tar opt/
 
     - name: Upload artifact
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       with:
         name: plumed-master
         path: ~/plumed-master.tar

.github/workflows/release.yml

@@ -22,7 +22,7 @@ jobs:
     - name: Build a binary wheel and a source tarball
       run: python3 -m build
     - name: Store the distribution packages
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       with:
         name: python-package-distributions
         path: dist/