The v1.2.0 release introduced PostgreSQL v18 as default, which causes
compatibility issues with existing v17 installations. This change:
- Sets default version back to v17 in values.yaml and schema
- Adjusts migration targetVersion to 38 to enable migration 37
- Migration 37 backfills version field for existing installations
This ensures existing PostgreSQL clusters continue working after upgrade.
Signed-off-by: IvanHunters <xorokhotnikov@gmail.com>