jenkins: Downgrade k8s plugin

Version 1.26.1 may be the latest, but it apparently has major breaking changes
that cause the jenkins operator to fail to schedule jobs. Unclear why, but
upstream operator suggests running 1.15.2. We'll run 1.15.10 to include some
security fixes and be "more up to date" rather than stale.

Change-Id: I06724c84197a5d7f5401ddb1499927d28a158879
diff --git a/k8s/mendel-linux.yaml b/k8s/mendel-linux.yaml
index f958e18..8e64141 100644
--- a/k8s/mendel-linux.yaml
+++ b/k8s/mendel-linux.yaml
@@ -9,7 +9,7 @@
     - name: authentication-tokens
       version: "1.4"
     - name: kubernetes
-      version: "1.26.1"
+      version: "1.15.10"
     - name: workflow-job
       version: "2.39"
     - name: workflow-aggregator