remove PHPBack (unmaintained since 2021) (#3784)

- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/issues/3558
- WARNING:awesome_lint.py: PHPBack: last updated -566 days, 12:18:06.178563 ago, older than 365 days

README.md

@@ -1608,7 +1608,6 @@ _Related: [Booking and Scheduling](#booking-and-scheduling)_
 - [Mobilizon](https://mobilizon.org) - A federated tool that helps you find, create and organise events and groups. ([Demo](https://demo.mobilizon.org/), [Source Code](https://framagit.org/framasoft/mobilizon/)) `GPL-3.0` `Elixir`
 - [OhMyForms](https://ohmyform.com/) - An open source alternative to TypeForm that can create stunning mobile-ready forms, surveys and questionnaires. ([Source Code](https://github.com/ohmyform/ohmyform)) `AGPL-3.0` `Docker`
 - [Open Event Server](https://github.com/fossasia/open-event-server) - Enables organizers to manage events from concerts to conferences and meet-ups. `GPL-3.0` `Python`
-- [PHPBack](https://www.phpback.org) - The open source feedback system. ([Demo](https://www.phpback.org/demo/), [Source Code](https://github.com/ivandiazwm/phpback)) `GPL-3.0` `PHP`
 
 
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