You can use one any of the good old [issue tracking systems](https://en.wikipedia.org/wiki/Comparison_of_issue-tracking_s...). AFAIK, Trac and Redmine are quite easy to run self-hosted, as long as you don't need to handle public projects (and therefore spam management).

Realistically, though, you're more likely to be able to convince other project members to use the issue tracker of whatever forge they're comfortable with, for instance Gitlab, Gitea, Pagure or Sourcehut.