Support separate issues index in delete_index rake task

What does this MR do?

Related to #273479 (closed)

Built upon work in: !48334 (merged) and !49370 (merged)

Note: Merging into the create_empty_index branch so that it will benefit from the new Helper method delete_standalone_indices

Update the gitlab:elastic:delete_index rake task so that it also deletes all the separate issues index.

Update specs & documentation (included a small update to the rake task description for gitlab:elastic:recreate_index since it now recreates multiple indexes by calling delete_index, then create_empty_index)

Screenshots (strongly suggested)

> bundle exec rake gitlab:elastic:delete_index
Index/alias 'gitlab-development' has been deleted
Index 'gitlab-development-issues-20201230-1039' with alias 'gitlab-development-issues' has been deleted

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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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