Create main branch by default for design repos

Currently when we copy design repo and the repo is empty
`master` branch is created by default.

Let's create `main` branch in this case instead

app/services/design_management/copy_design_collection/copy_service.rb

@@ -86,7 +86,7 @@ module DesignManagement
       def with_temporary_branch(&block)
         target_repository.create_if_not_exists
 
-        create_master_branch! if target_repository.empty?
+        create_default_branch! if target_repository.empty?
         create_temporary_branch!
 
         yield
@@ -95,9 +95,9 @@ module DesignManagement
       end
 
       # A project that does not have any designs will have a blank design
-      # repository. To create a temporary branch from `master` we need
-      # create `master` first by adding a file to it.
-      def create_master_branch!
+      # repository. To create a temporary branch from default branch we need to
+      # create default branch first by adding a file to it.
+      def create_default_branch!
         target_repository.create_file(
           git_user,
           ".CopyDesignCollectionService_#{Time.now.to_i}",
@@ -121,7 +121,7 @@ module DesignManagement
         target_repository.rm_branch(git_user, temporary_branch)
       end
 
-      # Merge the temporary branch containing the commits to `master`
+      # Merge the temporary branch containing the commits to default branch
       # and update the state of the target_design_collection.
       def finalize!
         source_sha = shas.last

app/services/design_management/design_service.rb

@@ -13,7 +13,7 @@ module DesignManagement
     attr_reader :issue
 
     def target_branch
-      repository.root_ref || "master"
+      repository.root_ref || Gitlab::DefaultBranch.value(object: project)
     end
 
     def collection

spec/services/design_management/copy_design_collection/copy_service_spec.rb

@@ -195,6 +195,14 @@ RSpec.describe DesignManagement::CopyDesignCollection::CopyService, :clean_gitla
             expect { subject }.to change { target_repository.branch_names }.from([]).to(['master'])
           end
 
+          it 'does not create default branch when one exists' do
+            target_repository.create_if_not_exists
+            target_repository.create_file(user, '.meta', '.gitlab', branch_name: 'new-branch', message: 'message')
+
+            expect { subject }.not_to change { target_repository.branch_names }
+            expect(target_repository.branch_names).to eq(['new-branch'])
+          end
+
           it 'leaves the design collection in the correct copy state' do
             subject