Advanced Search: Estimate Elasticsearch cluster size

What does this MR do?

Originally, we were going to create a new section in the Advanced Search settings page, but it requires us to add a lot of complexity for keeping the storage size up to date. Instead, we've decided to create a new rake task, which simplifies the task a lot.

$ bin/rake gitlab:elastic:estimate_cluster_size
This GitLab instance repository size is 828.3 MB.
By our estimates for such repository size, your cluster size should be at least 414.2 MB.
Please note that it is possible to index only selected namespaces/projects by using Elasticsearch indexing restrictions.

#221177 (closed)

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

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

Related to #221177 (closed)