Rename retention-config to retention in synapse additional config

- Use simpler property name 'retention' instead of 'retention-config'
- Helm Chart schema may not allow 'retention-config' naming convention

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

apps/production/custom-configs/synapse-values.yaml

@@ -23,7 +23,7 @@ data:
             - 'fe80::/10'
             - 'fc00::/7'
             max_spider_size: 10M
-        retention-config:
+        retention:
           config: |
             retention:
               enabled: true