docs(changelog): add deprecation warning to v1.2.0

CNPG operator in v1.2.0 updates default PostgreSQL image to v18
and cannot migrate from the previous major version automatically,
causing PostgreSQL clusters to fail after upgrade.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

Author: kvaps

docs/changelogs/v1.2.0.md

@@ -4,6 +4,8 @@ https://github.com/cozystack/cozystack/releases/tag/v1.2.0
 
 # Cozystack v1.2.0
 
+> **⚠️ WARNING: Do not use this release.** This version includes CloudNativePG operator, which updates the default PostgreSQL image to version 18. CNPG is unable to perform the migration from the previous major version automatically, which will cause PostgreSQL clusters to fail to start after the upgrade. Please use [v1.2.1](v1.2.1.md) instead.
+
 Cozystack v1.2.0 delivers significant platform enhancements: a fully managed **OpenSearch** service joining the application catalog, **VPC peering** for secure inter-tenant networking, tenant workload placement control via the new **SchedulingClass** system, a highly-available **VictoriaLogs cluster** replacing the single-node setup, and **Linstor volume relocation** for optimized clone and snapshot restore placement. Additional highlights include external-dns as a standalone extra package, multi-node RWX volume fixes, and a wave of dashboard and monitoring improvements.
 
 ## Feature Highlights