cubejs-elasticsearch-driver not compatible with v7.x

Describe the bug The backend package works with elasticsearch up to v6.8. With v7.x it causes the following error:

ResponseError: illegal_argument_exception
    at onBody (/cube/node_modules/@elastic/elasticsearch/lib/Transport.js:337:23)
    at IncomingMessage.onEnd (/cube/node_modules/@elastic/elasticsearch/lib/Transport.js:264:11)
    at IncomingMessage.emit (events.js:326:22)
    at endReadableNT (_stream_readable.js:1241:12)
    at processTicksAndRejections (internal/process/task_queues.js:84:21)

I've tested this behaviour both with elastic.io and a local vanilla elasticsearch v7.15 image. On the other hand, the library works correctly with elasticsearch v.6.8.18

To Reproduce Steps to reproduce the behavior:

• Deploy elasticsearch v7.15 using docker.
• Try to connect cube.js by following the instructions on the wiki
• Open the cube.js schema and see the error.

Expected behavior The lib should work with the latest version of elasticsearch