Use the current_user of the merge action to delete the source branch

Use the user of the merge action to perform the deletion of the source branch.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/497498

Epic: https://gitlab.com/groups/gitlab-org/-/epics/15714

To Test:

• Turn on feature Feature.enable(:switch_deletion_branch_user)
• Create an MR as user 1
• Merge as user 2
• Check in the user 2 activity, that they are deleted the branch
• Check in the user 1 activity, they are did not delete the branch