Add milestone_id to issue update graphQL mutation

- update documentation and specs

app/graphql/mutations/issues/update.rb

@@ -41,6 +41,11 @@ module Mutations
                required: false,
                description: 'The IDs of labels to be removed from the issue.'
 
+      argument :milestone_id,
+               GraphQL::ID_TYPE,
+               required: false,
+               description: 'The ID of the milestone to be assigned, milestone will be removed if set to null.'
+
       def resolve(project_path:, iid:, **args)
         issue = authorized_find!(project_path: project_path, iid: iid)
         project = issue.project

changelogs/unreleased/36312-issue-update-milestone.yml

+---
+title: Add milestone_id param to issue update graphQL mutation
+merge_request: 38684
+author:
+type: added

doc/api/graphql/reference/gitlab_schema.graphql

@@ -14736,6 +14736,11 @@ input UpdateIssueInput {
   """
   locked: Boolean
 
+  """
+  The ID of the milestone to be assigned, milestone will be removed if set to null.
+  """
+  milestoneId: ID
+
   """
   The project the issue to mutate is in
   """

doc/api/graphql/reference/gitlab_schema.json

@@ -43503,6 +43503,16 @@
               },
               "defaultValue": null
             },
+            {
+              "name": "milestoneId",
+              "description": "The ID of the milestone to be assigned, milestone will be removed if set to null.",
+              "type": {
+                "kind": "SCALAR",
+                "name": "ID",
+                "ofType": null
+              },
+              "defaultValue": null
+            },
             {
               "name": "healthStatus",
               "description": "The desired health status",

spec/graphql/mutations/issues/update_spec.rb

@@ -7,6 +7,7 @@ RSpec.describe Mutations::Issues::Update do
   let_it_be(:user) { create(:user) }
   let_it_be(:project_label) { create(:label, project: project) }
   let_it_be(:issue) { create(:issue, project: project, labels: [project_label]) }
+  let_it_be(:milestone) { create(:milestone, project: project) }
 
   let(:expected_attributes) do
     {
@@ -14,7 +15,8 @@ RSpec.describe Mutations::Issues::Update do
       description: 'new description',
       confidential: true,
       due_date: Date.tomorrow,
-      discussion_locked: true
+      discussion_locked: true,
+      milestone_id: milestone.id
     }
   end
   let(:mutation) { described_class.new(object: nil, context: { current_user: user }, field: nil) }
@@ -57,6 +59,16 @@ RSpec.describe Mutations::Issues::Update do
         end
       end
 
+      context 'when setting milestone to nil' do
+        let(:expected_attributes) { { milestone_id: nil } }
+
+        it 'changes the milestone corrrectly' do
+          issue.update_column(:milestone_id, milestone.id)
+
+          expect { subject }.to change { issue.reload.milestone }.from(milestone).to(nil)
+        end
+      end
+
       context 'when changing labels' do
         let_it_be(:label_1) { create(:label, project: project) }
         let_it_be(:label_2) { create(:label, project: project) }