github: workflows: docs: bump upload-artifact to v6

Bump the upload-artifact action from v5 to v6.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>

Author: henrikbrixandersen

.github/workflows/docs.yml

@@ -42,7 +42,7 @@ jobs:
 
       - name: Upload artifact
         if: github.event_name == 'pull_request'
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           path: doc/build/html