SAML auth uses legacy `_opendistro` route

[cwperks]

OpenSearch Security 2.1.0 release broke SAML authentication when a change in security-dashboards-plugin (opensearch-project/security-dashboards-plugin#895) to replace SAML routes from using legacy /_opendistro/_security/... to using /_plugins/_security was included without a corresponding change in this repo.

The change in security-dashboards-plugin has since been reverted (opensearch-project/security-dashboards-plugin#1035) and the documentation website was changed to refer back to the legacy urls (opensearch-project/documentation-website#877).

There is an open PR to support the /_plugins/_security/... route for SAML in this repo (#1936).

[peternied]

@cliu123 Can you make sure all the required changes go in together?

[cwperks]

[Triage] @cwperks Will open a PR with this change to solicit feedback.