ci: upgrade actions/setup-python to v6

Updates 2 occurrences across 2 workflow files from v5 to v6.
v6 runs on Node 24; v5 runs on Node 20 which is
being deprecated on GitHub Actions runners.

See https://github.com/actions/setup-python/releases/tag/v6.0.0

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: andrewhsu

.github/workflows/publish-apache-polaris-python-client.yml

@@ -37,7 +37,7 @@ jobs:
         with:
           repository: 'apache/polaris'
       - name: Set up Python 3.11
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version: 3.11
       - name: Generate client

.github/workflows/render-pr-body-release-notes.yml

@@ -22,7 +22,7 @@ jobs:
           curl -s -S -f -L -o rpchangelog/requirements.txt https://vectorized-public.s3.us-west-2.amazonaws.com/rpchangelog/requirements.txt
           curl -s -S -f -L -o rpchangelog/rpchangelog.py https://vectorized-public.s3.us-west-2.amazonaws.com/rpchangelog/rpchangelog.py
           chmod +x rpchangelog/rpchangelog.py
-      - uses: actions/setup-python@v5
+      - uses: actions/setup-python@v6
         with:
           python-version: '3.12'
           cache: 'pip'