Improve Duo-related copy in Duo Admin Area page

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/460121+.

This MR improves the way some of the Duo-related copy is worded, also in preparation for supporting all Duo products. See the issue referenced above for the exact changes.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Duo Tier	Before	After
Pro
Enterprise	same as above

Detailed screenshots of UI changes can be found in https://gitlab.com/gitlab-org/gitlab/-/issues/460121+.

How to set up and validate locally

Pre-testing steps

1. Start GDK, CDot, and ZSim.
2. Create a new testing user with a group.
3. Purchase an SM Ultimate subscription with a Duo Pro add-on for the group.

Testing Duo Pro

1. Navigate to the GitLab Duo admin page.
2. Confirm that the UI changes are as expected.

Testing Duo Enterprise

1. Navigate to the GitLab Duo admin page.
2. Spoof the Enterprise-tier add-on by doing the following:
   1. Apply this test.patch.
   2. Run the following commands in the Rails console:

# Undo all this after testing, if you'd like:
add_on = GitlabSubscriptions::AddOn.new(name: "duo_enterprise", description: "TEST Duo Enterprise").save!
add_on_purchase = GitlabSubscriptions::AddOnPurchase.last # or whatever gets you the target purchase
add_on_purchase.add_on = add_on
add_on_purchase.save!

1. Navigate to the GitLab Duo admin page.
2. Confirm that the UI changes are as expected.