Convert @arcgis/core/tasks to @arcgis/core/rest with v4.20

[steveoh]

import QueryTask from "@arcgis/core/tasks/QueryTask";
const task = new QueryTask({
  url: "url to service"
});
task.execute(new Query(...));

can now be written

import { execute } from "@arcgis/core/rest/query";
execute("url to service", new Query());

@arcgis/core/tasks will likely be removed from the API at some point.

[steveoh]

the query task is not used in atlas