ci: bump setup-python v5 -> v6 in setup-backend to run on Node 24

GitHub forces Actions off Node 20 starting June 16, 2026. The shared
setup-backend composite action still pins actions/setup-python@v5 (Node 20),
which surfaces a deprecation warning on every backend job. Bump to v6 (Node 24),
reusing the same pinned SHA already used in bump-python-package.yml.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/actions/setup-backend/action.yml

@@ -42,7 +42,7 @@ runs:
         fi
         echo "python-version=$RESOLVED_VERSION" >> "$GITHUB_OUTPUT"
     - name: Set up Python ${{ steps.set-python-version.outputs.python-version }}
-      uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
+      uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6
       with:
         python-version: ${{ steps.set-python-version.outputs.python-version }}
         cache: ${{ inputs.cache }}

.github/workflows/docker.yml

@@ -101,27 +101,13 @@ jobs:
             PUSH_OR_LOAD="--load"
           fi
 
-          # Retry to absorb transient Docker Hub registry errors (base-image
-          # pull timeouts, 504/401 on push, ECONNRESET) that otherwise fail
-          # the whole job. buildx reuses the buildkit layer cache from the
-          # failed attempt, so a retry mostly re-does just the failed push.
-          for attempt in 1 2 3; do
-            if supersetbot docker \
-              $PUSH_OR_LOAD \
-              --preset "$BUILD_PRESET" \
-              --context "$EVENT" \
-              --context-ref "$RELEASE" $FORCE_LATEST \
-              --extra-flags "--build-arg INCLUDE_CHROMIUM=false --tag $IMAGE_TAG" \
-              $PLATFORM_ARG; then
-              break
-            fi
-            if [ "$attempt" -eq 3 ]; then
-              echo "::error::supersetbot docker build failed after 3 attempts"
-              exit 1
-            fi
-            echo "::warning::Build attempt ${attempt} failed; retrying in 30s..."
-            sleep 30
-          done
+          supersetbot docker \
+            $PUSH_OR_LOAD \
+            --preset "$BUILD_PRESET" \
+            --context "$EVENT" \
+            --context-ref "$RELEASE" $FORCE_LATEST \
+            --extra-flags "--build-arg INCLUDE_CHROMIUM=false --tag $IMAGE_TAG" \
+            $PLATFORM_ARG
 
       # in the context of push (using multi-platform build), we need to pull the image locally
       - name: Docker pull