[DOCS] Replace 'elasticsearch' with 'opensearch' in variable, path and file names

[tlfeng]

Issue #, if available:
#238

Description of changes:

The PR continue the work in PR #247, because my original fork repository was deleted.

The PR requests merging the code into oss-docs branch.

• Replace all elasticsearch and es(all letters in lower case) that are used in variable, command, file and path names to opensearch
• Replace my_es_node with my_opensearch_node in "Get Index recovery API"
• Replace es_port with opensearch_port for a parameter in "discovery-gce" plugin
• Add TODO for a console output to be changed after renaming - // TODO(OpenSearch): Change the class path after renaming or test will be failed
• Add TODO for maven repository for the old Java API - // TODO(OpenSearch): Correct the maven repository

Exceptions:
Asciidoc variables (such as in { }, [[ ]], << >>, id=" ")
elasticsearch as plain text in code snippets
ES - letters in upper case
Website links
License

Testing: ./gradlew :docs:check passed

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[odfe-release-bot]

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[tlfeng]

Corrected the wrong placement mentioned in PR #247 (comment)

[odfe-release-bot]

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[odfe-release-bot]

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[odfe-release-bot]

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[odfe-release-bot]

❌   DCO Check Failed
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