Transition the Runner on Kubernetes overwrite pod spec feature to GA
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=556286)
</details>
<!--IssueSummary end-->
## Overview
Transition the [Kubernetes Overwrite generated pod specifications feature](https://docs.gitlab.com/runner/executors/kubernetes/#overwrite-generated-pod-specifications) to generally available.
## Background
1. The Kubernetes Overwrite generated pod specifications feature as released as an experimental feature on 2021-08-26. [MR][(https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/3114)
2. The feature transition to BETA in 16.8 (2024-01-16)
## Current [blockers](https://gitlab.com/gitlab-org/gitlab-runner/-/issues/38775#note_2571809226) for GA transition
- [ ] The PodSpec overwrite feature is only configurable in config.toml and lacks the security features needed to open it up to users in the `gitlab-ci.yaml`.
- [ ] Design document for API work.
issue