Adjust text and links for duo widget in sidebar for pivot

What does this MR do and why?

Adjusts text and links for duo widget in sidebar for pivot

• we've pivoted to just enabling Duo Core instead of the whole agent platform wording.
• see https://gitlab.com/gitlab-org/gitlab/-/issues/565473#note_2742739053
• see https://gitlab.com/gitlab-org/gitlab/-/issues/565473

Screenshots or screen recordings

Note Covered fully by tests

Screen_Recording_2025-09-10_at_1.09.59_PM

How to set up and validate locally

Note Covered fully by tests

See this

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Related to #565473