David Rutherford, the problem you're seeing is a problem in the artifactory plugin, not a problem in the git client plugin.
The artifactory plugin depends on JGit version 3. Git client plugin 2.x delivered JGit version 4 which was largely compatible with JGit version 3 but deprecates some API's that were part of JGit 3. Git client plugin 3.x will deliver JGit 5 which removes at least one of the deprecated APIs.
The artifactory plugin developers have created a pre-release of their plugin that uses the JGit 4 and JGit 5 API instead of the JGit 3 API.
Your best near term solution is to downgrade to git client plugin 2.7.x. The git client plugin 3.0.0-rc and git plugin 4.0.0-rc were intended to be release candidate builds for further testing in the experimental update center. They were not intended to be production releases for general availability. I made a serious mistake when I chose the "-rc" suffix for the release.
The update center will soon hide the git client plugin 3.0.0-rc and git plugin 4.0.0-rc release