Doc edits for moving Code Suggestions to Google Vertex AI Codey APIs

What does this MR do?

As decided by @sytses in the June 23, 2023 AI Strategy meeting, GitLab is considering Google AI as 1st Party AI Services.

This doc change removes mention of 3rd party AI for Code Suggestions, updates references to Google AI documentation, and attempts to make clear that "Google Vertex AI Codey APIs" are 1st party AI.

Related to: !124568 (merged)

Related issues

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