Allow setting the shard/replica separately for ES indexes (step 2)

What does this MR do?

This MR implements the UI for setting separate shard/replica settings for different Elasticsearch indices.

#296882 (closed)

Rollout plan

• Step 1: Create a new table and migrate existing index settings and change the backend to allow using this new table instead of application_settings. Done in !56344 (merged)
• Step 2: (we are here) Change the backend and the UI to allow changing settings for all existing indices. Ignoring no longer used fields (elasticsearch_replicas, elasticsearch_shards), in the application_settings table.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • [-] I have not included a changelog entry because _____.
• [-] 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 #296882 (closed)