Remove the legacy Contribution App and feature flag

What does this MR do and why?

Moving forward Contribution Analytics will always use GraphQL to lazy load the page data. This change removes the contribution_analytics_graphql feature flag, along with all of the now unused legacy code that was tied to it:

• Removes contents of legacy_components/*
• Removes group_members.js helper class for the legacy user table
• Cleans up the legacy code from data_collector.rb. This is also re-used by the GraphQL resolver, so I only removed the methods that are now obselete

How to set up and validate locally

Using the rails console, create a new Project. Using the ID, run the following:

project = Project.find(PROJECT_ID)
100.times { FactoryBot.create(:user, username: SecureRandom.hex, email: SecureRandom.hex + '@test.com').tap { |u| project.group.add_developer(u); Issues::CreateService.new(container: project, spam_params: nil, current_user: u, params: {title: 'title', description: 'description'}).execute } }
events = Event.where(project_id: project.id, action: 'created', target_type: 'Issue')
events.each { |i| i.update(created_at: rand(90).days.ago) }

1. Visit group contribution analytics for the new project: Ex http://gdk.test:3000/groups/flightjs/-/contribution_analytics
2. Validate that the mock data created is appearing on the page

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #395758 (closed)