Upgrading Elasticsearch version from 7.0.1 to 7.15 in Elasticsearch Adapter

The current Elasticsearch version in Calcite is 7.0.1, while the latest released ES version is 7.15.2.

There are many new SQL features added and bugs fixed from 7.0.1 to 7.15. By upgrading the embedded ES, we can make good use of ES's new features, especially about DSL, which enable developers to find one more suitable or effective way to implement features and improve existing functions.

Since low level rest client has good compatibility among 7.x versions, RestClient is also upgraded to 7.15.2, which is supported currently.