Gem cleanup and housekeeping

[william-lee-epigenesys]

• Drop support for Ruby 3.0
• Add Rubocop via epi-rubocop
• Format all files using rubocop -A
• Add rubocop as workflow job
• Clean up gemspec to only package necessary files, and resolve build warnings
• Update aruba_helper.rb to prepare temporary directory using fixtures system instead of running system commands
• Update year in LICENSE
• Run Ruby workflow on all PRs regardless of target branch