chore(deps): bump actions/download-artifact from 5 to 6

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

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/ci.yml

@@ -252,7 +252,7 @@ jobs:
           persist-credentials: false
 
       - name: Download libdeltachat.a
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: ${{ matrix.os }}-libdeltachat.a
           path: target/debug
@@ -313,7 +313,7 @@ jobs:
         run: pip install tox
 
       - name: Download deltachat-rpc-server
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: ${{ matrix.os }}-deltachat-rpc-server
           path: target/debug