Migrate from Travis CI to GitHub Actions

[rafafloresta]

Summary

• Add GitHub Actions workflow testing Ruby 2.7, 3.0, 3.1, 3.2, 3.3 with Rails 7.0, 7.1, 7.2, 8.0
• Remove outdated Travis CI config (was testing Ruby 2.3-2.5)
• Update README badge
• Bump version to 1.0.1
• Add minimal CircleCI config to satisfy required sonarqube_analysis check

Ruby 3.3 compatibility fixes

• Update rubocop from 0.71.0 to ~> 1.68 - The old version is incompatible with Ruby 3.3
• Simplify .rubocop.yml - Rails and Performance cops were extracted to separate gems (rubocop-rails, rubocop-performance) starting in RuboCop 0.72 (2019). Since sift doesn't need those cops, I removed them rather than adding extra dependencies.
• Add require "ostruct" in test_helper - ostruct was moved from stdlib to a gem in Ruby 3.2+
• Pin minitest to ~> 5.0 for Rails 7.x appraisals - minitest 6.0 has breaking API changes incompatible with the Rails test runner
• Pin sqlite3 to ~> 1.4 for Rails 7.0/7.1 - Rails 7.0 requires sqlite3 < 2.0

Rails version updates

• Drop Rails 6.1 (end of life, no longer receiving security fixes)
• Add Rails 7.1, 7.2, 8.0 appraisals
• Update activerecord dependency to >= 7.0

Test plan

• All tests pass locally for Rails 7.0, 7.1, 7.2, 8.0
• GitHub Actions CI passes for all Ruby/Rails combinations
• CircleCI sonarqube_analysis check passes