docs(openmetadata): warn about Elasticsearch searchType index update issue on restarts

[safayavatsal]

Summary

Ref #451

When using searchType: elasticsearch with Elasticsearch 8.x, the run-db-migrations init container fails on pod restarts with:

JsonpMappingException: Error deserializing PutMappingRequest: Unknown field 'settings'

Root cause: This is an application-level bug in ElasticSearchIndexManager.updateIndex() which sends the full index definition (including settings) to the Elasticsearch PutMapping API. The PutMapping endpoint only accepts mappings, not settings. This only occurs when updating existing indices — creating from scratch works fine. The default opensearch searchType is unaffected.

Since the fix requires changes to the OpenMetadata server (not the Helm chart), this PR adds documentation and warnings to help users avoid or work around the issue.

Changes

• values.yaml: Added warning comment above searchType explaining the limitation
• values.schema.json: Added description with warning; fixed default from "elasticsearch" to "opensearch" (was inconsistent with actual values.yaml default)
• README.md: Added inline note in the values table for searchType; added "Troubleshooting Elasticsearch Index Updates" section with 3 workarounds
• NOTES.txt: Added conditional warning shown at helm install/upgrade time when searchType: elasticsearch is detected

Workarounds documented

1. Switch to OpenSearch (recommended) — use the default searchType: opensearch
2. Delete indices before restart — forces fresh creation instead of update
3. Use the reindex CronJob — enable reindexConfig.enabled for periodic rebuilds

Test plan

• helm lint passes with default values (opensearch)
• helm lint passes with searchType: elasticsearch
• helm template renders cleanly for both searchType values
• NOTES.txt warning only appears when searchType: elasticsearch
• Verify README renders correctly on GitHub

[safayavatsal]

Taking up issue #451.

The root cause is in the OpenMetadata server's ElasticSearchIndexManager.updateIndex() which sends index settings inside a PutMappingRequest — Elasticsearch 8.x rejects this as an unknown field. This only happens on pod restarts when indices already exist (update path), not on fresh creation.

Since the proper fix belongs in the OpenMetadata server repo, this PR adds documentation, warnings, and workaround guidance at the Helm chart level. Also fixes a pre-existing inconsistency where values.schema.json had default: "elasticsearch" while values.yaml defaults to "opensearch".