Docs: clarify model selection for code review features

What does this MR do?

The first in a series of MRs to address confusion reported by a customer over setting a model for Code Review Flow in GitLab 18.11.

Context:

• Before 19.1, Code Review Flow used the same model setting as the classic GitLab Duo Code Review, so they shared the same default model and available models. As a result, Code Review Flow customers on those older versions don't get the latest Code Review Flow models through the AI Gateway, they are restricted to whatever is available for GitLab Duo Code Review. Our current docs don't make that clear.
• This thread has more details and a clearer outline of what I'm trying to address in this MR: #614057 (comment 3741211458)

This MR:

• adds information about how to select a model for Code Review Flow on Self-Managed, depending on which version of GitLab you're using
• updates some outdated UI labels in the steps for selecting models
• makes a few updates to indicate that the default and available models are different for Code Review Flow if you're on GitLab 19.0 or earlier
• highlights that changing the setting impacts both code review versions if you're on GitLab 19.0 or earlier.

Other MRs planned for this work item:

• Renaming the setting in the UI to "Non-Agentic Code Review" to be more explicit and mirror the "Agentic Code Review" setting that already exists.
• Update the history items for model changes to use dates instead of milestones.

Additional item out of scope for this work item:

• Creating a table of models with dates of when they are added, deprecated, and removed, see #624428

Related issues

Docs: Clarify model selection settings for clas... (#614057 - closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.