build(deps): bump actions/upload-artifact from 4 to 7

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 7.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](https://github.com/actions/upload-artifact/compare/v4...v7)

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

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/snap-build.yml

@@ -63,7 +63,7 @@ jobs:
           echo "snap=${SNAP}" >> "${GITHUB_OUTPUT}"
 
       - name: Upload .snap artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: etherpad-snap-pr-${{ github.event.pull_request.number || github.run_id }}
           path: ${{ steps.artifact.outputs.snap }}

.github/workflows/snap-publish.yml

@@ -42,7 +42,7 @@ jobs:
         uses: snapcore/action-build@v1
 
       - name: Upload snap artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: etherpad-snap
           path: ${{ steps.build.outputs.snap }}