ci: bump actions/download-artifact from 6 to 7

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 6 to 7.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v6...v7)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '7'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/main.yml

@@ -93,7 +93,7 @@ jobs:
           # needed to avoid MPI warning
           echo "OMPI_MCA_btl=^openib" >> $GITHUB_ENV
     - name: Download artifacts with plumed
-      uses: actions/download-artifact@v6
+      uses: actions/download-artifact@v7
       with:
         name: plumed-master
         path: ~

.github/workflows/release.yml

@@ -42,7 +42,7 @@ jobs:
 
     steps:
     - name: Download all the dists
-      uses: actions/download-artifact@v6
+      uses: actions/download-artifact@v7
       with:
         name: python-package-distributions
         path: dist/