Fix Style/OpenStructUse offenses omniauth_saml_spec

What does this MR do and why?

This MR aims to solve the issue #344279 (closed)

Fixes OpenStruct use since Ruby 3 discourages it.

File Change

• .rubocop_todo/style/open_struct_use.yml
• spec/dependencies/omniauth_saml_spec.rb

MR acceptance checklist

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