[WIP] Fix integration test and bump Ruby/Rails version for test

[nard-tech]

• Replace Bundler.with_clean_env with Bundler.with_unbundled_env in integration tests
  • cf. https://github.com/ctran/annotate_models/runs/2219100430#step:5:8
• Bump Rails version to latest version (5.2.5 and 6.1.3.1)
  • cf. https://weblog.rubyonrails.org/2021/3/26/marcel-upgrade-releases/
• Bump Ruby versions
  • remove 2.4, 2.5
  • add 2.7, 3.0
  • cf. https://www.ruby-lang.org/en/news/2020/04/05/support-of-ruby-2-4-has-ended/