Upgrade ActiveRecord to 7.2 and bump version to 0.6.0

[krhitoshi]

Summary

This PR upgrades ActiveRecord from 7.1 to 7.2 and bumps the version to 0.6.0.

Changes

• ActiveRecord: Upgrade from ~> 7.1.0 to ~> 7.2.0 (7.2 series only)
• Ruby version requirement: Update to >= 3.1.0, < 3.5
  • Rails 7.2 requires Ruby 3.1.0 or newer
  • Drop support for Ruby 2.7 and 3.0
  • Update README to reflect Ruby 3.1 - 3.4 support
• CI matrix: Update to test Ruby 3.1, 3.2, 3.3, 3.4 (remove Ruby 2.7, 3.0)
• Dockerfile: Update base image from Ruby 2.7 to Ruby 3.1
• Dockerfile: Remove bundler version pin (no longer needed for Ruby 3.1+)
• Version: Bump to 0.6.0
• README: Add version 0.6.x note indicating Active Record 7.2 base

Technical Details

• The dependency is pinned to ~> 7.2.0 to use only the 7.2 series
• This installs the latest ActiveRecord 7.2.x release
• Rails 7.2 is the current stable release with ongoing support
• Ruby 3.1 is the minimum required version for Rails 7.2

Test Plan

• CI will verify compatibility with Ruby 3.1, 3.2, 3.3, 3.4
• All existing tests should pass with ActiveRecord 7.2
• Verify bundle install completes successfully with ActiveRecord 7.2.x
• Docker build should succeed with Ruby 3.1 base image