Add reliable tag to milestone e2e specs

qa/qa/specs/features/browser_ui/2_plan/milestone/assign_milestone_spec.rb

 # frozen_string_literal: true
 
 module QA
-  RSpec.describe 'Plan' do
+  RSpec.describe 'Plan', :reliable do
     describe 'Milestones' do
       include Support::Dates
 

qa/qa/specs/features/browser_ui/2_plan/milestone/create_group_milestone_spec.rb

 # frozen_string_literal: true
 
 module QA
-  RSpec.describe 'Plan' do
+  RSpec.describe 'Plan', :reliable do
     describe 'Group milestone' do
       include Support::Dates
 

qa/qa/specs/features/browser_ui/2_plan/milestone/create_project_milestone_spec.rb

 # frozen_string_literal: true
 
 module QA
-  RSpec.describe 'Plan' do
+  RSpec.describe 'Plan', :reliable do
     describe 'Project milestone' do
       include Support::Dates