Fix create_migrations_index for ES6

What does this MR do?

This MR fixes an exception for Elasticsearch 6 (ES7 works fine and we test against it in our specs) when we're trying to create a migrations index.

[1] pry(main)> Gitlab::Elastic::Helper.default.create_migrations_index
Elasticsearch::Transport::Transport::Errors::BadRequest: [400] {"error":{"root_cause":[{"type":"mapper_parsing_exception","reason":"Root mapping definition has unsupported parameters:  [completed : {type=boolean}]"}],"type":"mapper_parsing_exception","reason":"Failed to parse mapping [properties]: Root mapping definition has unsupported parameters:  [completed : {type=boolean}]","caused_by":{"type":"mapper_parsing_exception","reason":"Root mapping definition has unsupported parameters:  [completed : {type=boolean}]"}},"status":400}
from /home/dgruzd/.asdf/installs/ruby/2.7.2/lib/ruby/gems/2.7.0/gems/elasticsearch-transport-6.8.2/lib/elasticsearch/transport/transport/base.rb:208:in `__raise_transport_error'

QA pipeline that caught this error: https://gitlab.com/gitlab-org/gitlab-qa-mirror/-/jobs/847304449

Reported in: #281934 (comment 447152555)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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