chore(deps): bump actions/upload-artifact from 4 to 6 (#787)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: SnO₂WMaN <me@sno2wman.net>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -73,7 +73,7 @@ jobs:
           lake exe graph ./import_graph.html
 
       - name: Upload lean artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: lean-artifacts
           path: |