Remove custom backgrounds from flash alerts

What does this MR do and why?

Flash alerts were using a specific color variable which was mixing styles in dark mode. This change removes the unique styling from the .scss and migrate a few lingering flash alerts to Pajamas components.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Light mode (no visual change)

Before	After

Dark mode

Before	After

How to set up and validate locally

1. Trigger a flash alert by creating an issue to resolve a discussion thread in a merge request

2. Terms of service (can be added via http://gdk.test:3000/admin/application_settings/general#js-terms-settings)

3. In the milestone overview if you have more than 500 issues warning (http://gdk.test:3000/flightjs/Flight/-/milestones/6#tab-issues)

   Heres a script you can use in the rails console to generate it

   # Replace these with your actual project ID and milestone ID
   project_id = 7  # Your project ID
   milestone_id = 35  # Your milestone ID
   
   # Find the project and milestone by ID
   project = Project.find(project_id)
   milestone = Milestone.find(milestone_id)
   
   # Use the first user in the database, or you could find a specific user by ID
   user = User.first
   
   600.times do |i|
     result = Issues::CreateService.new(
       container: project,
       current_user: user,
       params: {
         title: "Issue #{i+1}",
         description: "This is the description for issue #{i+1}",
         milestone_id: milestone.id
       }
     ).execute
   
     if result.success?
       issue = result.payload[:issue]
       puts "Created issue ##{i+1}: #{issue.iid} (ID: #{issue.id})"
     else
       puts "Failed to create issue ##{i+1}: #{result.message}"
     end
   
     sleep 0.1  # Small delay to avoid overwhelming the system
   end
   
   puts "Finished creating issues."
   
   # Print a summary
   total_issues = project.issues.count
   milestone_issues = milestone.issues.count
   puts "Total issues in project: #{total_issues}"
   puts "Issues assigned to milestone: #{milestone_issues}"

4. Compare light and dark mode