Drop support Ruby 2.6 and Rails 6

[Bhacaz]

This pull request introduces several updates to modernize the codebase and improve CI/CD workflows. The main changes include upgrading Ruby and Rails dependencies, enhancing the GitHub Actions setup to support multiple Ruby versions and add Rubocop linting, and updating gem dependencies for better compatibility and functionality.

CI/CD Workflow Improvements:

• Upgraded the GitHub Actions workflow to run RSpec tests across Ruby versions 2.7 to 3.4, switched to ubuntu-24.04, updated actions/checkout to v4, and replaced the build step with RSpec execution. Added a separate Rubocop job for linting on Ruby 3.4. (.github/workflows/main.yml)

Dependency and Compatibility Updates:

• Increased the minimum required Ruby version to 2.7.0 in the gemspec and updated dependencies to require actionmailer and railties version 7.0 or higher, replacing the previous rails dependency. (gravity_mailbox.gemspec) [1] [2]
• Added new gems to the Gemfile for broader compatibility and functionality, including base64, bigdecimal, concurrent-ruby, drb, mutex_m, and net-http.

Linting and Testing Enhancements:

• Updated .rubocop.yml to use the plugins key instead of require for Rubocop configuration, improving compatibility with newer Rubocop versions.
• Removed an unnecessary Rubocop directive (rubocop:disable RSpec/Rails/InferredSpecType) from the mailbox controller spec for cleaner test code. (spec/controllers/gravity_mailbox/mailbox_controller_spec.rb)