Add/document Support for OpenSearch

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Is your feature request related to a problem? Please describe.

Coming from opensearch-project/OpenSearch#12456, a user is asking for FSCrawler support for OpenSearch. I found 3 issues that mention varying issues/successes with using OpenSearch instead of Elasticsearch as a back-end.

Describe the solution you'd like

0. Understanding what's broken/not working. Does anyone know?
1. Having code/integration tests for OpenSearch 1.x and 2.x.
2. Clearly documenting which version(s) of Elasticsearch and OpenSearch are supported.
3. Changing H1 in README from "File System Crawler for Elasticsearch" to "File System Crawler for Elasticsearch and OpenSearch".

Describe alternatives you've considered

Forking FSCrawler for OpenSearch support.