chore: CI: bump actions/download-artifact from 5 to 6 (#11053)

Bumps
[actions/download-artifact](https://github.com/actions/download-artifact)
from 5 to 6.

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -375,7 +375,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: build
     steps:
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
         with:
           path: artifacts
       - name: Release
@@ -407,7 +407,7 @@ jobs:
           # Doesn't seem to be working when additionally fetching from lean4-nightly
           #filter: tree:0
           token: ${{ secrets.PUSH_NIGHTLY_TOKEN }}
-      - uses: actions/download-artifact@v5
+      - uses: actions/download-artifact@v6
         with:
           path: artifacts
       - name: Prepare Nightly Release