Properly validate milestones param in releases API

The same way we already do that for POST method

changelogs/unreleased/277369-typeerror-can-t-quote-activesupport-hashwithindifferentaccess.yml

+---
+title: Fix milestones param validation for releases API PUT method
+merge_request: 47169
+author:
+type: fixed

lib/api/releases.rb

@@ -91,7 +91,7 @@ module API
         optional :name,        type: String, desc: 'The name of the release'
         optional :description, type: String, desc: 'Release notes with markdown support'
         optional :released_at, type: DateTime, desc: 'The date when the release will be/was ready.'
-        optional :milestones,  type: Array, desc: 'The titles of the related milestones'
+        optional :milestones,  type: Array[String], coerce_with: ::API::Validations::Types::CommaSeparatedToArray.coerce, desc: 'The titles of the related milestones'
       end
       put ':id/releases/:tag_name', requirements: RELEASE_ENDPOINT_REQUIREMENTS do
         authorize_update_release!

spec/requests/api/releases_spec.rb

@@ -998,6 +998,17 @@ RSpec.describe API::Releases do
           end
         end
 
+        context 'without milestones parameter' do
+          let(:params) { { name: 'some new name' } }
+
+          it 'does not change the milestone' do
+            subject
+
+            expect(response).to have_gitlab_http_status(:ok)
+            expect(returned_milestones).to match_array(['v1.0'])
+          end
+        end
+
         context 'multiple milestones' do
           context 'with one new' do
             let!(:milestone2) { create(:milestone, project: project, title: 'milestone2') }