Upgrade ActiveRecord to 8.1 and bump version to 0.8.0

[krhitoshi]

Summary

This PR upgrades ActiveRecord from 8.0 to 8.1 and bumps the version to 0.8.0.

Changes

• ActiveRecord: Upgrade from ~> 8.0.0 to ~> 8.1.0 (8.1 series only)
• Ruby version requirement: Update from >= 2.7.0, < 3.5 to >= 3.2
  • Rails 8.1 requires Ruby 3.2.0 or newer (same as Rails 8.0)
  • Update README to reflect "Ruby 3.2 or newer"
  • Simplify gemspec requirement to >= 3.2
• Dependencies:
  • Update thor from ~> 1.3.1 to ~> 1.4.0
  • Update terminal-table from ~> 3.0.2 to ~> 4.0.0
• Version: Bump to 0.8.0
• README: Add version 0.8.x note indicating Active Record 8.1 base

Technical Details

• The dependency is pinned to ~> 8.1.0 to use only the 8.1 series
• This installs the latest ActiveRecord 8.1.x release
• Rails 8.1 is the latest stable release with ongoing support
• Ruby 3.2 is the minimum required version for Rails 8.1

Test Plan

• CI will verify compatibility with Ruby 3.2, 3.3, 3.4
• All existing tests should pass with ActiveRecord 8.1
• Verify bundle install completes successfully with ActiveRecord 8.1.x
• Docker build should succeed with Ruby 3.2 base image