[Docs/Ops] Update Code Suggestions docs, runbooks, alerts and self-hosted tooling for v4

Summary

Align documentation, runbooks, alerts, and self-hosted tooling with /v4/code/suggestions as the primary Code Suggestions endpoint and clearly mark v2/v3 as deprecated.

Scope

  • Update AI Gateway API docs and GitLab deprecations to mark v2/v3 endpoints as deprecated and point to v4 and the migration path.
  • Update testing guides and design docs (sequence/routing diagrams) to show /v4/code/suggestions on the hot path.
  • Update Code Suggestions runbooks plus Mimir/Prometheus alerting and recording rules so they primarily track /v4/code/suggestions, keeping v2/v3 matchers only as log-only during overlap and removing them when cleanup is done.
  • Update the self-hosted troubleshoot_selfhosted_installation.py script to probe /v4/code/suggestions and document the minimum AI Gateway version that fully supports v4 and which GitLab self-managed versions bundle it.

Acceptance criteria

  • Docs and diagrams reflect v4 as the primary endpoint, with v2/v3 clearly marked as deprecated.
  • Runbooks and SLI/alert rules focus on /v4/code/suggestions and still provide coverage during the overlap period.
  • The troubleshoot script validates v4-capable AI Gateway instances successfully, and the minimum AIGW version for v4 is documented and linked from relevant GitLab self-hosted docs.