Use Recommended AI-native Terminology in UI copy

What does this MR do and why?

Updates "AI-powered" terminology to "AI-native" in UI copy and documentation, bringing it in line with recommended usage in the GitLab language style guide.

Out of Scope:

• Updating usage in URLs
• Updating internal references e.g "AI-powered stage"
• Updating existing marketing blogs or posts that already use the term "AI-powered"

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Related to #537400 (closed)