StreamReceiver article to be replaced by describing a custom element request handler

[mshabarov]

Tell us about your request

StreamReceiver article to be renewed to avoid this low-level Flow API (deprecated soon). Looks useless because it doesn't describe how to implement SteramVariable.

Should instead describe how to make a custom upload or download handler with vaadin/flow#21164.

---

If you think this issue is important, add a 👍 reaction to help the community and maintainers prioritize this issue.