Migration of existing Watcher alerts to Alerting Framework

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Current State

At present, Stack Monitoring has six alerts which ship with the application. These are as follows:

1. Cluster status

2. Nodes changed

3. Elasticsearch version mismatch

4. Kibana version mismatch

5. Logstash version mismatch

6. License expiration

As one can plainly see, these are defined and shipped with Elasticsearch and use the Watcher framework.

Desired State

The Kibana team is rapidly developing a new alerting framework. .

We want to take our existing watches and migrate them to the new framework. This issue serves as a meta-issue to track the progress of that effort.

1. Cluster status
2. Nodes changed
3. Elasticsearch version mismatch
4. Kibana version mismatch
5. Logstash version mismatch
6. License expiration

cc: @elastic/stack-monitoring

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Closing in favor of a meta-issue elsewhere.