Complete Rails 8.0.4 support

[takaokouji]

Add full Rails 8.0 compatibility while maintaining backward compatibility with Rails 6.1, 7.0, 7.1, and 7.2.

Key Changes:

1. SQLite3 Dependency Update:

   • Rails 8.0 requires sqlite3 >= 2.1 (was ~> 1.4)
   • Add conditional gem requirement in Gemfile:
     • Rails 8.x: sqlite3 >= 2.1
     • Rails 6-7: sqlite3 ~> 1.4
   • Use bundle lock --add-platform ruby for cross-platform support

2. Schema Format Configuration:

   • Rails 8.0 removed :none as valid schema_format option
   • Add version-conditional logic in test_helper.rb:
     • Rails 8.0+: Use :ruby format
     • Rails < 8.0: Use :none format (disables schema dumps)

Files Modified:

• Gemfile: Added Rails 8.x sqlite3 version condition
• test/test_helper.rb: Conditional schema_format based on Rails version

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
• Rails 8.0.4: ✅ 674/674 tests passing

All backward compatibility maintained across Rails 6.1-8.0.

🤖 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