Instance- or group-wide setting for remove_source_branch_after_merge
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### Problem to solve
With our [implementation](https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/27939) of [project-level default configuration](https://gitlab.com/gitlab-org/gitlab/issues/18283) for "Remove Source Branch After Merge", it is desirable to allow administrators to choose their preferred default setting for newly-created projects.
### Intended users
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
### Proposal
As proposed in the [original community-contributed issue](https://gitlab.com/gitlab-org/gitlab/issues/18283), a group- or instance- wide setting to allow administrators to choose their preferred default would satisfy this use case.
### Permissions and Security
### Documentation
### Testing
### What does success look like, and how can we measure that?
### What is the type of buyer?
### Links / references
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