jenkins: Update to 2.375.1

I don't want Jenkins updating itself whenever the pod restarts, so I'm
going to pin it to a specific version.  This way, I can be sure to take
a snapshot of the data volume before upgrading.

jenkins/jenkins.yaml

@@ -117,8 +117,8 @@ spec:
     spec:
       containers:
       - name: jenkins
-        image: docker.io/jenkins/jenkins:lts
-        imagePullPolicy: Always
+        image: docker.io/jenkins/jenkins:2.375.1-lts
+        imagePullPolicy: IfNotPresent
         ports:
         - name: http
           containerPort: 8080