Proposal to mark etcd-io/dbtester as public archive

Sub issue under https://github.com/etcd-io/etcd/issues/16367.

This etcd sub-project has been inactive for a significant period, with the last meaningful activity occurring in 2018. Given the lack of recent contributions, I propose marking this repository as a public archive.

This action would clearly communicate to the community that the project is no longer actively maintained.

**Reasoning:**
1. **Inactivity:** The repository has not seen any substantial activity for the past five years.

2. **Clarity:** Marking the repository as a public archive would provide clarity to users and potential contributors. It indicates that the project is in a dormant state and that any forks or contributions would likely not be integrated.

3. **Resource Allocation:** Keeping inactive repositories in the active organization may create confusion and require unnecessary resource allocation responding to issues etc.

**Next steps**

I kindly request @etcd-io/maintainers-etcd and previous [main subproject contributor](https://github.com/etcd-io/dbtester/graphs/contributors) @gyuho review the proposal and please provide feedback.