Based on this (screenshot of the 2.6 documentation) the recommendation is to use Fleet if you want something that is already integrated into Rancher manager.
That stated, I've had good luck deploying Jenkins into a downstream cluster, and then using the same cluster as a "Cloud" (see screenshot) for Terraform runners. I put my IAC in Github, with a Rancher2 provider used for cluster deployments. Terraform state files are stored as Kubernetes secrets in the same cluster where Jenkins is deployed (you can also use an S3 bucket or a secrets vault... but this is a lab environment). Actual Kubernetes apps can then be handled by ArgoCD.
But that is no longer natively integrated into Rancher. Hope that makes sense.