[Backport 2.x] Fixed a bug to prevent updating index.knn setting after index creation

[anntians]

Description

This change prevents customers to update index.knn setting after creating an index. Previously, there was a loophole to migrate a live k-nn index from hot to UW by closing the index, setting index.knn to false, opening the index, and migrating to UW. However, migrating k-nn indices to UW nodes is not supported. Thus, this PR updates index.knn to be FINAL and immutable after index creation.

Related Issues

Resolves #2334

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[kotwanikunal]

@anntians Can you please fix spotless?

You can use '--warning-mode all' to show the individual deprecation warnings and determine if they come from your own scripts or plugins.
Execution failed for task ':qa:restart-upgrade:spotlessJavaCheck'.
> The following files had format violations:
      src\test\java\org\opensearch\knn\bwc\IndexingIT.java
          @@ -6,7 +6,6 @@
           package�org.opensearch.knn.bwc;
           
           import�org.junit.Assert;
          -import�org.opensearch.client.Response;
           import�org.opensearch.client.ResponseException;
           import�org.opensearch.common.settings.Settings;
           import�org.opensearch.common.xcontent.XContentFactory;