Elasticsearch 7.6.1 compatibility

[ftianli-amzn]

Goal
Prepare release for OpenDistro 1.6.0.0 with ElasticSearch 7.6.1

Major Dependency Updates
ES 7.4.2 -> 7.6.1
Gradle 5.6.2 -> 6.2.2
JDK 12 -> 13

Issues
There is an undocumented breaking changes since ES 7.5 :
elastic/elasticsearch#47572

1. It influenced JaCoCo integration for nearly all OpenDistro plugins.
2. It influenced launching multi-node clusters locally for alerting plugin.
   https://github.com/opendistro-for-elasticsearch/alerting#advanced-launching-multi-node-clusters-locally

[ftianli-amzn]

https://github.com/ftianli-amzn/alerting/tree/opendistro-1.6
Currently, I commented out all the incompatible codes to make it build successfully. The team is still working on to find a good solution.

Update 1: JaCoCo integration has been fixed.
Update 2: All problems have been fixed, and waiting for the release.