Remove support for ES6

[rodrigozhou]

What changed? Removing support for Elasticsearch 6.

Why? Elasticsearch 6 has reached end of life earlier this year.

How did you test it? Removed unit/integration tests related to ES6, and ran existing unit tests and integration tests.

Potential risks It won't work with clients using ES6.

Is hotfix candidate? No.

[rodrigozhou]

There are also the schema/elasticsearch/visibility/versioned/v*/index_template_v6.json files. Can I delete them too?

[yiminc]

Yes, remove index_template_v6.json, and also cluster_settings_v6.json.

[rodrigozhou]

Removed the index templates for ES6, and also updated schema scripts.