Add non_archived param for merge requests api

The param default value is true, returning
merge requests from non archived projects only.

changelogs/unreleased/28352-group-merge-request-api-searches-archived-projects.yml

+---
+title: Add non_archived param to group merge requests API endpoint to filter MRs from non archived projects
+merge_request: 23809
+author:
+type: added

doc/api/merge_requests.md

@@ -397,6 +397,7 @@ Parameters:
 | `source_branch`                 | string         | no       | Return merge requests with the given source branch                                                                             |
 | `target_branch`                 | string         | no       | Return merge requests with the given target branch                                                                             |
 | `search`                        | string         | no       | Search merge requests against their `title` and `description` |
+| `non_archived`                  | Boolean        | no       | Return merge requests from non archived projects only. Default is true. _(Introduced in [GitLab 12.8](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/23809))_  |
 
 ```json
 [

lib/api/merge_requests.rb

@@ -141,6 +141,8 @@ module API
       end
       params do
         use :merge_requests_params
+        optional :non_archived, type: Boolean, desc: 'Return merge requests from non archived projects',
+        default: true
       end
       get ":id/merge_requests" do
         merge_requests = find_merge_requests(group_id: user_group.id, include_subgroups: true)

spec/requests/api/merge_requests_spec.rb

@@ -807,6 +807,38 @@ describe API::MergeRequests do
         end
       end
     end
+
+    context 'with archived projects' do
+      let(:project2) { create(:project, :public, :archived, namespace: group) }
+      let!(:merge_request_archived) { create(:merge_request, title: 'archived mr', author: user, source_project: project2, target_project: project2) }
+
+      it 'returns an array excluding merge_requests from archived projects' do
+        get api(endpoint_path, user)
+
+        expect_response_contain_exactly(
+          merge_request_merged,
+          merge_request_locked,
+          merge_request_closed,
+          merge_request
+        )
+      end
+
+      context 'with non_archived param set as false' do
+        it 'returns an array including merge_requests from archived projects' do
+          path = endpoint_path + '?non_archived=false'
+
+          get api(path, user)
+
+          expect_response_contain_exactly(
+            merge_request_merged,
+            merge_request_locked,
+            merge_request_closed,
+            merge_request,
+            merge_request_archived
+          )
+        end
+      end
+    end
   end
 
   describe "GET /projects/:id/merge_requests/:merge_request_iid" do