WIP: Step 2: Support multiple indices to Elasticsearch Admin options

What does this MR do?

Implements the new Elasticsearch administration UI, and backend support for properly handling multiple indices.

This branch consists of the following MRs which have been reviewed separately:

Issue	MR	Description
#13533 (closed)	!15736 (closed)	Move Elasticsearch admin options to top level Note: es-admin-step2 was branched off this MR, which is why it isn't showing up as merged
#12555 (closed)	!15275 (merged)	Add ElasticsearchIndex model
#13534 (closed), #32055 (closed)	!18254 (merged)	ES administration API and multiple indices support
#13534 (closed), #32056 (closed)	!20421 (merged)	ES administration Vue app
#12712 (closed)	!18661 (merged)	Remove feature flag for ES web indexing
#118478 (closed)		Rake tasks Note: This needs to be addressed before this MR can be merged

Pre-release checklist

Before this MR can be merged into master, the following tasks need to be addressed:

• Rebase against master, resolve any conflicts, fix any CI failures.
• Adress potentially blocking follow-up issues in &1769, specifically:
  • Finalize docs: #35418 (closed)
  • Finalize QA tests: #42684 (closed)
  • Snippets web indexing: #13641 (closed)
  • Retrying logic: #38111 (closed)
  • Caching logic: #38109 (closed)
• Revert change in GITLAB_ELASTICSEARCH_INDEXER_VERSION (needs to be >1.5.0)
• Update GitLab release version ("introduced in ...")
  • ee/lib/api/elasticsearch_indices.rb
  • doc/api/elasticsearch_indices.md
• Add changelog entry.
• Perform a final DB maintainer review.
• Perform a final BE maintainer review.
• Perform a final FE maintainer review.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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