Update UI text for MR pipeline rename

What does this MR do and why?

Resolves #351193 (closed)

As part of the related issue, we're renaming "Pipelines for merge requests" and "Pipelines for merged results" to simply:

• Merge request pipelines
• Merged results pipelines

This updates the UI to match. In fact, in many places I found we already used the updated name, so there were only a few remaining places that needed to be update. See screenshots for details. Docs are being handled separately, in !79197 (merged).

Screenshots or screen recordings

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