Add [Enable "Delete source branch" option by default] on group level
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### Problem to solve
I cannot set "Enable "Delete source branch" option by default" on a group-level
### Intended users
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#delaney-development-team-lead)
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
### User experience goal
Similar checkbox option "Enable "Delete source branch" option by default" like it already exists on project-level (Settings -> MR -> Merge Options) having a small hint that it this option will override the setting of all subgroups/projects
### Proposal
Add option "Enable "Delete source branch" option by default" on a group-level, so the setting will be inherited to all subgroups/projects
### Further details
After migrating groups and projects from several sources into gitlab.com I now have the issue that this setting is not consistent across 200+ projects. Only way to resolve this currently (besides using the API) is to click through 200+ projects.
### Permissions and Security
* [x] Add expected impact to Maintainer (40) members
* [x] Add expected impact to Owner (50) members
issue