docs(upgrade): replace NEXT placeholder with v2.0.1
Fix docs/UPGRADE.md migration matrix header: ## v2.0.0 → NEXT → ## v2.0.0 → v2.0.1. The UPGRADE guide is referenced by the 2.0.1 changelog for deployers (mandatory PG16 migration).
Fix docs/UPGRADE.md migration matrix header: ## v2.0.0 → NEXT → ## v2.0.0 → v2.0.1. The UPGRADE guide is referenced by the 2.0.1 changelog for deployers (mandatory PG16 migration).