Change notes removal migration to use ES tasks

What does this MR do?

Related to #297646 (closed)

The Notes migration has run (and completed) in staging and production but I observed some issues with it that I wanted to fix.

Slack discussion: https://gitlab.slack.com/archives/C3TMLK465/p1620848477178800

This change adds two new parameters to the DeleteByQuery Elasticsearch API call based on the discussion I had with the Search team:

• conflicts: proceed - We did see some conflicts and this will instruct Elasticsearch to continue instead of failing the request.
• wait_for_completion: false - This will create a task which we can use to track the completion of the call. The task must be cleaned up from the Elasticsearch tasks index (it is not done automatically).

The specs have been updated to reflect the changes.

Note: This will not cause the migration to run again in staging or production

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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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