jenkins: Use gerrit plugin instead of git

The git ModernSCM plugin has been deprecated in favor of gerrit.

Change-Id: Ief4959fcbc01c1b33753fea45ce4d5809412fb18
diff --git a/k8s/configmap.yaml b/k8s/configmap.yaml
index 89ed191..c276b0b 100644
--- a/k8s/configmap.yaml
+++ b/k8s/configmap.yaml
@@ -19,7 +19,7 @@
             retriever:
               modernSCM:
                 scm:
-                  git:
+                  gerrit:
                     remote: https://coral.googlesource.com/gke-jenkins
     # The jenkins: stanza must be the last one in this list for credentials to work!
     jenkins: