Add requries_admin meta to mentions_spec

qa/qa/specs/features/browser_ui/2_plan/issue/mentions_spec.rb

 # frozen_string_literal: true
 
 module QA
-  RSpec.describe 'Plan', :smoke, :reliable do
+  # TODO: Remove :requires_admin meta when the `Runtime::Feature.enable` method call is removed
+  RSpec.describe 'Plan', :smoke, :reliable, :requires_admin do
     describe 'mention' do
       let(:user) { Resource::User.fabricate_or_use(Runtime::Env.gitlab_qa_username_1, Runtime::Env.gitlab_qa_password_1) }
       let(:project) do

qa/qa/specs/features/ee/browser_ui/2_plan/issue_boards/read_only_board_configuration_spec.rb

 # frozen_string_literal: true
 
 module QA
-  RSpec.describe 'Plan', :reliable do
+  # TODO: Remove :requires_admin meta when the `Runtime::Feature.enable` method call is removed
+  RSpec.describe 'Plan', :reliable, :requires_admin do
     describe 'Read-only board configuration' do
       let(:qa_user) do
         Resource::User.fabricate_or_use(Runtime::Env.gitlab_qa_username_1, Runtime::Env.gitlab_qa_password_1)

qa/qa/specs/features/ee/browser_ui/2_plan/multiple_assignees_for_issues/four_assignees_spec.rb

 # frozen_string_literal: true
 
 module QA
-  RSpec.describe 'Plan', :reliable do
+  # TODO: Remove :requires_admin meta when the `Runtime::Feature.enable` method call is removed
+  RSpec.describe 'Plan', :reliable, :requires_admin do
     describe 'Multiple assignees per issue' do
       before do
         Flow::Login.sign_in

qa/qa/specs/features/ee/browser_ui/2_plan/multiple_assignees_for_issues/more_than_four_assignees_spec.rb

 # frozen_string_literal: true
 
 module QA
-  RSpec.describe 'Plan', :reliable do
+  # TODO: Remove :requires_admin meta when the `Runtime::Feature.enable` method call is removed
+  RSpec.describe 'Plan', :reliable, :requires_admin do
     describe 'Multiple assignees per issue' do
       let(:project) do
         Resource::Project.fabricate_via_api! do |project|