Replace _opendistro route to _plugins

[willyborankin]

Description

The change is related to opensearch-project/security#5066 in the security plugin, which removes _opendisto route

What is the old behavior before changes and new behavior after changes?

Issues Resolved

[List any issues this PR will resolve (Is this a backport? If so, please add backport PR # and/or commits #)]

Testing

[Please provide details of testing done: unit testing, integration testing and manual testing]

Check List

• New functionality includes testing
• New functionality has been documented
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.46%. Comparing base (600ab91) to head (e646f4b).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2172   +/-   ##
=======================================
  Coverage   71.46%   71.46%           
=======================================
  Files          97       97           
  Lines        2649     2649           
  Branches      403      403           
=======================================
  Hits         1893     1893           
  Misses        641      641           
  Partials      115      115           

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[cwperks]

Thank you @willyborankin. I just merged in the latest changes from main which had fixes for all CI checks. I expect most checks to pass now, except for the SAML tests for this PR.

[willyborankin]

@cwperks as you expected only SAML tests failed

[DarshitChanpura]

@willyborankin I believe this assertion builder needs to change in security plugin before we merge this:
https://github.com/opensearch-project/security/blob/34735bb44a85c4e3f74fb666103b2d9670ce2ac6/src/main/java/com/amazon/dlic/auth/http/saml/Saml2SettingsProvider.java#L221

[willyborankin]

@willyborankin I believe this assertion builder needs to change in security plugin before we merge this: https://github.com/opensearch-project/security/blob/34735bb44a85c4e3f74fb666103b2d9670ce2ac6/src/main/java/com/amazon/dlic/auth/http/saml/Saml2SettingsProvider.java#L221

Closing this, as all paths will be removed in version 4.x."