Replace references to /_opendistro with /_plugins

[jotok]

Is your feature request related to a problem? Please provide an existing Issue # , or describe.
Updating calls to the Performance Analyzer plugin API to follow the new /_plugins naming convention.

Describe the solution you are proposing
RCA behavior should be unchanged.

Describe alternatives you've considered
This is a required change to follow new OpenSearch guidelines.

Additional context
https://github.com/opensearch-project/opensearch-plugins/blob/main/UPGRADING.md#settings-backwards-compatibility

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[dblock]

This all executes client-side, right? There's nothing server-side that needs to preserve backward compatibility here?

[jotok]

@dblock The RCA app creates a server that responds to the requests on port 9600, which means we do need to make a change for backward compatibility. We will follow up this PR with one that adds backward compatibility for the /_opendistro resource. We want to merge this PR first to unblock work on perftop.