Breaking changes from 6.xedit

Moving from types to typeless APIs in Elasticsearch 7.0edit

Elasticsearch 7.0 deprecated APIs that accept types, introduced new typeless APIs, and removed support for the default mapping. Read [this](https://www.elastic.co/blog/moving-from-types-to-typeless-apis-in-elasticsearch-7-0) blog post for more information.

Type hint and return typeedit

Added type hints and return type declarations in all the code base, where possible. See PR [#897](https://github.com/elastic/elasticsearch-php/pull/897).

PHP 7.1+ Requirementedit

We require using PHP 7.1+ for elasticsearch-php. PHP 7.0 is not supported since 1st Jan 2019. See [PHP supported version](https://www.php.net/supported-versions.php) for more information.