remove Re:Backlogs (unmaintained since 2020) (#3565)

- `WARNING:awesome_lint.py: Re:Backlogs: last updated -935 days, 9:53:39.976670 ago, older than 365 days`
- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/issues/3558

README.md

@@ -1869,7 +1869,6 @@ _See also: [awesome-sysadmin/Code Review](https://github.com/awesome-foss/awesom
 - [Phorge](https://we.phorge.it/) - Phorge is an open source, community driven platform for collaborating, managing, organizing and reviewing software development projects. ([Source Code](https://we.phorge.it/source/phorge/)) `Apache-2.0` `PHP`
 - [Phproject](https://www.phproject.org/) - High performance full-featured project management system. ([Source Code](https://github.com/Alanaktion/phproject)) `GPL-3.0` `PHP`
 - [ProjeQtOr](https://www.projeqtor.org/) - A complete, mature, multi-user project management system with extensive functionality for all phases of a project. ([Demo](https://demo.projeqtor.org/), [Source Code](https://sourceforge.net/p/projectorria/code/HEAD/tree/branches/)) `AGPL-3.0` `PHP`
-- [Re:Backlogs](https://github.com/kaishuu0123/rebacklogs) - Project management and collaboration tool powered by Ruby on Rails & VueJS. ([Demo](https://rebacklogs.saino.me/users/sign_up)) `MIT` `Ruby`
 - [Redmine](https://www.redmine.org/) - Redmine is a flexible project management web application. ([Demo](http://demo.redmine.org/), [Source Code](https://svn.redmine.org/redmine/)) `GPL-2.0` `Ruby`
 - [RhodeCode](https://rhodecode.com/) - RhodeCode is an open source platform for software development teams. It unifies and simplifies repository management for Git, Subversion, and Mercurial. ([Source Code](https://code.rhodecode.com/)) `AGPL-3.0` `Python`
 - [SCM Manager](https://www.scm-manager.org/) - The easiest way to share and manage your Git, Mercurial and Subversion repositories over http. ([Source Code](https://github.com/scm-manager/scm-manager)) `BSD-3-Clause` `Java`