Recently at work we've done an analysis of a of CI/CD tools. Many of them are self-hosted, free (and open source) and support a variety of workflows - including pull request related checks (either by polling of via webhooks). A cursory search will yield you a lot to play with, so... go ahead and do that.
That said, if you don't want to think about it too much, you can't go wrong with Jenkins.
Some people will suggest GitLab. I'd steer clear of GitLab, though, because of their operational incompetence [1] and their funny ideas about mandatory corporate espionage [2] - I mean, telemetry - which they only backed away from because people yelled at them. That second one was enough to disqualify them in our analysis (the first one just cemented the idea).
[1] https://about.gitlab.com/blog/2017/02/10/postmortem-of-datab...
[2] https://www.theregister.co.uk/2019/10/30/gitlab_backtracks_o...