drop Rails 7.1; add Rails 8.1 and raise Rails minimum to 7.2

[fumihumi]

Changes

• Remove Rails 7.1 gemfile: .github/gemfiles/rails_7.1.gemfile
• Add Rails 8.1 gemfile: .github/gemfiles/rails_8.1.gemfile
• CI matrix (.github/workflows/rspec.yml):
  • Rails: ["7.2","8.0","8.1"] (7.1 removed, 8.1 added)
  • Ruby: ["3.2","3.3","3.4"] (unchanged; 3.2 kept)
• Raise Rails dependency minimums in octoball.gemspec (ActiveRecord/ActiveSupport >= 7.2)
• Keep Ruby requirement at >= 3.2.0
• Remove legacy shim: lib/octoball/connection_handling.rb

[flfymoss]

Looks good to merge :)