🏠 Housekeeping

• Eliminate double CI builds on PRs (#2035) @mattbrictson
• Update gemspec with correct release notes URL, etc. (#2036) @mattbrictson
• Automatically maintain release notes using GitHub Actions (#2034) @mattbrictson
• Use a stub for $stdin during testing (#2033) @mattbrictson
• Upgrade nokogiri to fix security warning (#2031) @mattbrictson
• Exclude danger to fix Ruby 2.0.0 CI error (#2030) @mattbrictson