hdbcli: Fix wrong connection argument name (#10070)

The name should be `user` and not `username`

stubs/hdbcli/hdbcli/dbapi.pyi

@@ -18,7 +18,7 @@ class Connection:
         self,
         address: str,
         port: int,
-        username: str,
+        user: str,
         password: str,
         autocommit: bool = ...,
         packetsize: int | None = ...,