WA-RAILS7-008: Update ActiveSupport::Deprecation.new for Rails 7.1+

[kitcommerce]

Summary

In Rails 7.1, ActiveSupport::Deprecation.new was changed — the global deprecation instance was removed and per-gem deprecators are preferred. Workarea creates its own deprecation instance:

# core/lib/workarea.rb:183
@deprecation ||= ActiveSupport::Deprecation.new('3.6', 'Workarea')

This works on Rails 7.0 but the API changed in 7.1 where ActiveSupport::Deprecation behavior differs.

Approach

Use ActiveSupport::Deprecation._instance pattern for Rails < 7.1, and Rails.application.deprecators[:workarea] for Rails 7.1+.

Objective

Make Workarea's deprecation system work on Rails 7.1+.

Client impact

None — internal deprecation tooling.

Acceptance Criteria

• Deprecation warnings emit correctly on both Rails 6.1 and 7.1
• Tests pass

Verification Plan

1. Trigger a known deprecation warning and verify output format
2. Run test suite