[package testing] Decrease timeout to 2 hours (#104668)

Package testing on average takes slightly over an hour.  In error
cases where tests retry or a process hangs, it makes sense to keep this
timeout closer to the testing time.

Ref: https://kibana-ci.elastic.co/job/elastic+kibana+package-testing/

.ci/package-testing/Jenkinsfile

@@ -1,7 +1,7 @@
 #!/bin/groovy
 library 'kibana-pipeline-library'
 kibanaLibrary.load()
-kibanaPipeline(timeoutMinutes: 300) {
+kibanaPipeline(timeoutMinutes: 120) {
   slackNotifications.onFailure {
     ciStats.trackBuild {
       workers.ci(ramDisk: false, name: "package-build", size: 'l', runErrorReporter: false) {