Context:

• feat(sink): introduce Retract sink type #23593
• fix(iceberg): enfore iceberg sink type to retract when specified as upsert #23697

Currently for non-append-only iceberg sink, we call it "upsert" in both user-facing part and internal implementation. However, to be more accurate, non-append-only iceberg sink requires equality deletes, i.e., retractable stream in our system.

We'd better clarify the term for it.