Add upcoming_milestones helper

This commit introduces a new method to get upcoming milestones from
a given date. This will be used to schedule migration in n+3 milestone.

keeps/helpers/milestones.rb

@@ -15,6 +15,10 @@ def past_milestone(milestones_ago:)
         milestones[current_milestone_index + milestones_ago].version
       end
 
+      def upcoming_milestones
+        milestones.select { |milestone| Date.parse(milestone.date).future? }.reverse
+      end
+
       private
 
       def current_milestone

spec/keeps/helpers/milestones_spec.rb

@@ -124,4 +124,17 @@
       it { is_expected.to eq(true) }
     end
   end
+
+  describe '#upcoming_milestone', time_travel_to: '2024-04-17' do
+    subject(:upcoming_milestones) { described_class.new.upcoming_milestones }
+
+    it 'returns milestones in the future' do
+      expected_milestones = [
+        described_class::Milestone.new(version: '16.11', date: '2024-04-18'),
+        described_class::Milestone.new(version: '17.0', date: '2024-05-16')
+      ]
+
+      expect(upcoming_milestones).to contain_exactly(*expected_milestones)
+    end
+  end
 end