pipeline management hard code version in metadata

[kaisecheng]

Not sure if it should classify as a bug because users have no impact by now. However, this impact Logstash behavior in pipelines comparison

Kibana version:
master

Elasticsearch version:
master

Server OS version:
mac

Browser version:
chrome Version 86.0.4240.111 (Official Build) (x86_64)

Browser OS version:
mac

Describe the bug:
The centralized pipeline management overwrite pipeline["pipeline_metadata"]["version"] to "1" in every save action.
User can do all the actions but the "version" information lose in elasticsearch

Steps to reproduce:

1. create a new pipeline heartbeat in Kibana

2. update the new pipeline in elasticsearch with "version": "2"

curl -X PUT -v "localhost:9200/_logstash/pipeline/heartbeat"  -H 'Content-Type: application/json' -d '{
    "pipeline": "input { generator { count => 100 }} output { stdout { codec => \"rubydebug\"}}",
    "last_modified": "2020-10-01T15:42:30.229Z",
    "pipeline_metadata": { "version": "2"},
    "username": "log.stash",
    "pipeline_settings": {"pipeline.batch.delay": "50"}
}'

3. edit the same pipeline heartbeat in Kibana and save the change

4. fetch pipeline setting in elasticsearch

curl -X GET -u "elastic:elasticpass" "localhost:9200/_logstash/pipeline/heartbeat"

you will see the version is "1" again.

Expected behavior:
Kibana is expected to update "version" in every save action

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)