Grey out "Delete source branch" option for specific branches

Problem to solve

• Along gitlab-foss#33728 (closed), it would be nice to have the ability to disable the "Delete source branch" option when merging to a specific branch

Intended users

• Sasha (Software Developer)

Further details

Depending on the flow, having this option disabled for some specific branches would let the software developer never delete his branch when merging to a testing branch.

Proposal

Per project, add the option in the Project Settings.

Permissions and Security

Being a project Maintainer.

What does success look like, and how can we measure that?

From what the maintainer select, it should not do the opposite:

• From the Project settings, if the maintainer disabled the ability for a software developer to create a MR with this disabled by default, it should not delete the branch.