Delete merge requests from original ES index

What does this MR do?

Last step in #322800 (closed)

The merge requests have already been moved over to the a separate index (!60586 (merged)) and searches are verified to be using the new separate index.

This MR will remove all merge requests from the original index freeing up space 💪 Spec added to verify migration fails/completes appropriately.

How to test

Verify that you have:

• Advanced Search setup and enabled (for searching and indexing)
• Data including merge requests loaded

Open a rails console and run the migration worker. You can follow the migration progress in the elasticsearch.log file.

Elastic::MigrationWorker.new.perform

Screenshots (strongly suggested)

elasticsearch.log from a successful migration

{"severity":"INFO","time":"2021-05-10T17:38:39.876Z","correlation_id":null,"message":"MigrationWorker: migration[DeleteMergeRequestsFromOriginalIndex] executing migrate method"}
{"severity":"INFO","time":"2021-05-10T17:38:39.894Z","correlation_id":null,"message":"[Elastic::Migration: 20210510113500] Checking to see if migration is completed based on index counts remaining:3747"}
{"severity":"INFO","time":"2021-05-10T17:38:41.647Z","correlation_id":null,"message":"[Elastic::Migration: 20210510113500] Checking to see if migration is completed based on index counts remaining:0"}
{"severity":"INFO","time":"2021-05-10T17:38:41.655Z","correlation_id":null,"message":"[Elastic::Migration: 20210510113500] Checking to see if migration is completed based on index counts remaining:0"}
{"severity":"INFO","time":"2021-05-10T17:38:41.655Z","correlation_id":null,"message":"MigrationWorker: migration[DeleteMergeRequestsFromOriginalIndex] updating with completed: true"}

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.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

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
• Security reports checked/validated by a reviewer from the AppSec team