Update Duo Self-Hosted copy to reflect instance-level model selection

What does this MR do and why?

Instance-level (admin) model selection is consolidated within the Duo Self-Hosted UI. As a result, we need to update existing copies to reflect Duo Self-Hosted customers that also have instance level model selection enabled:

• Duo Self-Hosted link out card
• Duo Self-Hosted page description

References

Update Copy of Duo self-hosted Link-out Card fo... (#567707 - closed)

Screenshots or screen recordings

Duo Self-Hosted page description:

With no online license/FF disabled	With online license/FF enabled

Duo Self-Hosted link out card:

With no online license/FF disabled	With online license/FF enabled	Namespace model selection not affected

How to set up and validate locally

• Run GDK on non-SASS mode: GITLAB_SIMULATE_SAAS=0 gdk start
• Run the following setup script to get Duo Enterprise add on: GITLAB_SIMULATE_SAAS=0 bundle exec 'rake gitlab:duo:setup'
• Enable/disable the instance level model selection FF: /rails/features/instance_level_model_selection
• Validate description:
  • Go to /admin/gitlab_duo/self_hosted/features
  • Observe copy based on flag value
• Validate link out card:
  • Go to /admin/gitlab_duo
  • Observe different copy for Duo Self-Hosted link out card based on flag value

MR acceptance checklist

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