Complete Rails 7.2.3 support

[takaokouji]

Add full Rails 7.2 compatibility while maintaining backward compatibility with Rails 6.1, 7.0, and 7.1.

Key Changes:

1. Deprecation API Updates (Rails 7.2 made methods private):

   • Add JSONAPI.warn_deprecated() helper method
   • Replace all ActiveSupport::Deprecation.warn calls
   • Handle ActiveSupport::Deprecation.silenced= conditionally in tests
   • Falls back to Kernel#warn with [DEPRECATION] prefix for Rails 7.2+

2. Configuration Initialization Fix:

   • Change from attr_accessor to custom getter method
   • Ensures @configuration ||= Configuration.new works reliably
   • Fixed "undefined method for nil" errors in 304 tests

3. Test Fixture Path Changes:

   • Rails 7.2 changed fixture_path= to fixture_paths= (array)
   • Add conditional handling in three test classes:
     • Minitest::Test
     • ActiveSupport::TestCase * ActionDispatch::IntegrationTest

4. Test Helper Deprecation Silencing:

   • Update test_helper.rb to handle Rails 7.2 deprecators API
   • Conditional logic for silencing deprecation warnings

Files Modified:

• lib/jsonapi/configuration.rb: Added warn_deprecated helper + fixed getter
• lib/jsonapi/basic_resource.rb: Use JSONAPI.warn_deprecated
• lib/jsonapi/acts_as_resource_controller.rb: Use JSONAPI.warn_deprecated
• lib/jsonapi/relationship.rb: Use JSONAPI.warn_deprecated
• test/test_helper.rb: Fixture paths + deprecation silencing
• test/unit/resource/resource_test.rb: Conditional silenced= handling
• test/integration/requests/request_test.rb: Conditional silenced= handling

Test Results:

• Rails 6.1.7.10: ✅ 674/674 tests passing
• Rails 7.0.10: ✅ 674/674 tests passing
• Rails 7.1.6: ✅ 674/674 tests passing
• Rails 7.2.3: ✅ 674/674 tests passing

🤖 Generated with Claude Code

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
• My submission passes all tests. (Please run the full test suite locally to cut down on noise from travis failures.)
• I've used Github auto-closing keywords in the commit message or the description.
• I've added/updated tests for this change.

New Feature Submissions:

• I've submitted an issue that describes this feature, and received the go ahead from the maintainers.
• My submission includes new tests.
• My submission maintains compliance with JSON:API.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions