Undo type:ignore: legitimate SA2.0 error (see note)

In 2.0 the type of result is ChunkedIteratorResult, which does not have
`rowcount` attribute. In 1.4, it's LegacyCursorResult, which DOES have a
rowcount. HOWEVER, as per SA docs:
"Statements that use RETURNING may not return a correct rowcount."
https://docs.sqlalchemy.org/en/20/core/connections.html#sqlalchemy.engine.CursorResult.rowcount

So, this whole code block needs to be redone.

lib/galaxy/celery/base_task.py

@@ -103,7 +103,7 @@ def calculate_task_start_time(  # type: ignore
             result = sa_session.execute(
                 self._update_stmt, {"userid": user_id, "interval": task_interval_secs, "now": now}
             )
-            if result.rowcount == 0:  # type:ignore[attr-defined]
+            if result.rowcount == 0:
                 sched_time = now + datetime.timedelta(seconds=task_interval_secs)
                 result = sa_session.execute(
                     self._upsert_stmt, {"userid": user_id, "now": now, "sched_time": sched_time}