[SAML] Deprecate the legacy prefix of _opendistro in SAML flow

[RyanL1997]

Description
We should replace the SAML usage of legacy prefix _opendistro with _plugin at the next major version.

How can one reproduce the bug?

• /_opendistro/_security/saml/acs (source code)
• /_opendistro/_security/saml/logout in documentation of dashboards configuration

Exit Criteria
Replace the above paths with new prefix at first slug - _plugin

Do you have any additional context?
A potential fix + migration of /_opendistro/_security/saml/acs:

• Step 1: Security frontend changes to support new ACS url: [Enhancement] Add support for SAML ACS url with new prefix of _plugin security-dashboards-plugin#1565
• Step 2: Security backend changes to register ACS url with new prefix: [WIP] Change the saml acs endpoint to support _plugin  #3246
• Step 3: Dashboard core change to modify the xsrf.allowList if the user input the legacy url : opensearch-project/OpenSearch-Dashboards@main...RyanL1997:OpenSearch-Dashboards:xsrfallowlist-update

Useful links

1. Setting up ACS url in SAML: [Enhancement] Add support for SAML ACS url with new prefix of _plugin security-dashboards-plugin#1565 (comment)

[stephen-crawford]

[Triage] Thank you @RyanL1997 for filing this issue. This is part of the overall work to remove the deprecated nomenclature from the next major version.

[cwperks]

Closing as a duplicate of #2060