Draft: feat: add OS deprecation warning banner

Relates to issue #517674

Changes

This MR implements an OS deprecation warning banner system for GitLab administrators to alert them when their operating system is approaching end-of-life.

Core Components

OS Support Checker Service (lib/gitlab/os_support_checker.rb)

• Uses Ohai gem to detect OS platform and version
• Reads EOL data from supported_os_data.yml
• Calculates months until EOL date
• Returns status hash with OS information and support status
• Handles version matching (exact and major version)

Admin OS Support Helper (app/helpers/admin/os_support_helper.rb)

• show_os_deprecation_banner? - Shows banner only to admins when EOL ≤6 months away
• os_banner_variant - Returns :warning for 6 months, :danger for ≤3 months
• os_banner_dismissible? - Returns false when ≤1 month until EOL
• os_banner_message - Generates appropriate message based on time until EOL

Banner View Partial (app/views/admin/dashboard/_os_deprecation_banner.html.haml)

• Uses Users::DismissibleAlertComponent when dismissible
• Uses Pajamas::AlertComponent when non-dismissible (≤1 month)
• Applies correct variant (warning/danger) based on time until EOL

Integration Points

Dashboard Integration (app/views/admin/dashboard/index.html.haml)

• Banner rendered on admin dashboard after web_ide_oauth_callback_mismatch_callout

Controller (app/controllers/admin/dashboard_controller.rb)

• Assigns @os_support_status from OS Support Checker
• Includes Admin::OsSupportHelper

Callout System

• Added os_deprecation_banner: 120 enum to app/models/users/callout.rb
• Added OS_DEPRECATION_BANNER constant to app/helpers/users/callouts_helper.rb

Banner Behavior

1. ✅ No banner shown if EOL is more than 6 months away
2. ✅ Orange (warning) dismissible banner if EOL is 6 months away
3. ✅ Red (danger) dismissible banner if EOL is 3 months away
4. ✅ Red (danger) non-dismissible banner if EOL is 1 month away
5. ✅ Only shown to administrators (current_user.can_admin_all_resources?)

Test Coverage

• OS Support Checker Specs - Tests OS detection, YAML parsing, EOL calculations, error handling
• Helper Specs - Tests all helper methods for various scenarios
• View Specs - Tests banner rendering for different time periods and user types
• Controller Specs - Tests @os_support_status assignment
• Callout Model Specs - Tests enum value

Additional Files

Sample OS Data File (supported_os_data.yml)

• Contains sample EOL data for Ubuntu, Debian, AlmaLinux, Rocky, CentOS, RHEL, Amazon Linux, openSUSE Leap, and SLES
• Serves as reference/example (actual file will be provided by omnibus-gitlab)