[core] Deprecates append-row operation on RowList, replaced by multivalue-append.
Prevents null inference issues.

```java
import acolyte.RowLists;
import acolyte.Rows;

// Before
RowLists.rowList2(String.class, Integer.class).
  append(Rows.row2("str", 1)); // now deprecated

RowLists.rowList2(String.class, Integer.class).
  append("str", 1);
```
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