[FF] code_completion_v4_endpoint -- Rollout of migration to AIGW v4 endpoint for code completions
## Summary
This issue is to roll out [the feature](https://gitlab.com/gitlab-org/gitlab/-/work_items/598413) on production, that is currently behind the `code_completion_v4_endpoint` feature flag on Gitlab.
## Owners
* Most appropriate Slack channel to reach out to: `#g_ai_coding`
* Best individual to reach out to: @mikewronski
## Expectations
### What are we expecting to happen?
All code completions through Gitlab.com API will go to AI Gateway v4 `/v4/code/suggestions` instead of `/v2/code/completions` .
There is unlikely to be any noticeable change in completion behaviour as the v4 endpoint is now on parity with v2 completions with slight variations in underlying implementation.
### What can go wrong and how would we detect it?
* Code completions may not work correctly, may be delayed or output may be of lower quality
* Monitor error rates in the [Code Suggestions telemetry dashboard](https://log.gprd.gitlab.net/app/dashboards#/view/6c947f80-7c07-11ed-9f43-e3784d7fe3ca?_g=%28refreshInterval:%28pause:!t,value:0%29,time:%28from:now-6h,to:now%29%29)
## Rollout Steps
### Rollout on non-production environments
* [ ] Enable the feature flag on staging: `/chatops run feature set code_completion_v4_endpoint true --dev --pre --staging --staging-ref`
* [ ] Verify that the feature works as expected.
### Specific rollout on production
* [ ] `/chatops gitlab run feature set --feature-group=gitlab_team_members code_completion_v4_endpoint true`
* [ ] `/chatops run feature set --project=gitlab-org/gitlab code_completion_v4_endpoint true`
* [ ] Verify that the feature works.
### Global rollout on production
* [ ] Incrementally roll out the feature on production.
* [ ] After the feature has been 100% enabled, wait for at least one day.
### Release the feature
* [ ] Create a merge request to remove the `code_completion_v4_endpoint` feature flag.
* [ ] Close this rollout issue.
## Rollback Steps
* [ ] `/chatops run feature set code_completion_v4_endpoint false`
issue