Remove ES5 support and make it work with ES7

1. Remove CI tests for v5 since we don't support it anymore
2. Remove the ability to run against v5 by taking out the index.mapping.single_type logic
3. Update CI to test against 6.8.6,6.4.2,7.5.1 . It's important to test against 6.4.2 as well because 6.8 introduced new features to be forward compatible with 7.x
4. Remove the standard token filter since it is not available in ES 7 and according to the docs it does nothing anyway
5. Add support for ES7 by setting include_type_name when creating the index. This will ensure ES7 behaves like ES6 when creating mappings. See https://www.elastic.co/blog/moving-from-types-to-typeless-apis-in-elasticsearch-7-0 for more information.

Equivalent changes for GitLab at gitlab!22859 (merged)

gitlab#118662 (closed)