Jenkins Git Client Plugin Issue
Solutions:

You have a few choices to resolve the security issue in git client plugin 3.0.0-rc.

1) Install git plugin 3.12.1 and git client plugin 2.8.5 from the Jenkins update center
2) Upload git plugin 3.12.1 and git client plugin 2.8.6 into your Jenkins server from the "Advanced" tab of the "Manage Plugins" page inside "Manage Jenkins".
3)Install git plugin 4.0.0-beta11 and git client plugin 3.0.0-beta11 from the Jenkins experimental update center.
4)Upload git plugin 4.0.0-beta11 and git client plugin 3.0.0-beta11 into your Jenkins server from the "Advanced" tab of the "Manage Plugins" page inside "Manage Jenkins"

If you choose git plugin 3.12.1, you are choosing the released git plugin and the most stable.

If you choose git plugin 4.0.0-beta11, you are choosing the most recent pre-release git plugin with more bug fixes than are available in git plugin 3.12.1.

Reference: Git Client Plugin

GitHub Link

Plugin manual download

Related Issue : https://issues.jenkins-ci.org/browse/JENKINS-59336