Remove support for Elasticsearch 6.7 and prior

What concerns do we have for Self-managed customers if we removed support for Elasticsearch 6.x

When would gitlab.com move to Elasticsearch 8.0? UPDATE: Now Sept 2021 Release Date.

Steps to be compatible with 8.x

Good news per #247929 (comment 414788625) is that we'd already set up our mappings in a mostly compatible way for ES 8. Some small steps will be required to remove the redundant _type we are sending though.

1. Upgrade client libraries
2. Remove this _type being sent in bulk requests: https://gitlab.com/gitlab-org/gitlab/-/blob/94ca7fd72f3ed89e85458135bb6f05675b4b7e9b/ee/lib/gitlab/elastic/bulk_indexer.rb#L78
3. Remove this type being set in Go indexer https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/-/blob/0ff0da3c0981f193929e6f82d3249b415fd5bf68/elastic/client.go#L192
4. Remove this document type being set for searches https://gitlab.com/gitlab-org/gitlab/-/blob/94ca7fd72f3ed89e85458135bb6f05675b4b7e9b/ee/lib/elastic/class_proxy_util.rb#L15
5. There are also references to document_type in https://gitlab.com/gitlab-org/gitlab/-/blob/94ca7fd72f3ed89e85458135bb6f05675b4b7e9b/ee/lib/gem_extensions/elasticsearch/model/indexing/instance_methods.rb but I have a feeling this is all dead code since we should only ever be indexing documents using our custom bulk indexing code.
6. Change nGram to ngram since it was renamed after 7.6 and the old name is removed in 8 https://gitlab.com/gitlab-org/gitlab/-/blob/94ca7fd72f3ed89e85458135bb6f05675b4b7e9b/ee/lib/elastic/latest/config.rb#L84