CI: Add Ruby 4.0 and Rails 8.1 to CI Matrix

[taketo1113]

This Pull Request adds Ruby 4.0 and Rails 8.1 to the CI Matrix.

It also includes the following changes:

• Fix a load error for the benchmark gem on Rails 6.1 with Ruby 4.0
• Remove Ruby 3.2 from CI Matrix following the rails-dev (Rails 8.2) support drop
• Bump actions/checkout from 4 to 6

Additional Information

Fix Load Error of benchmark for Rails 6.1 with Ruby 4.0

In Ruby 4.0, benchmark gem was changed from a default gem to a bundled gem, which causes the following load error on Rails 6.1.
To fix this issue, the benchmark gem has been added as a dependency for the Rails 6.1 appraisal.

An error occurred while loading ./spec/lib/saml_idp/name_id_formatter_spec.rb.
Failure/Error: Bundler.require(*Rails.groups(:assets => %w(development test)))

Bundler::GemRequireError:
  There was an error while trying to load the gem 'saml_idp'.
  Gem Load Error is: cannot load such file -- benchmark
  Backtrace for gem load error is:
  /Users/taketo/.rbenv/versions/4.0.0/lib/ruby/4.0.0/bundled_gems.rb:60:in 'Kernel.require'

CI: Drop support for Ruby 3.2 rails-dev (rails 8.2)

Rails 8.2 will drop support for Ruby 3.2, so Ruby 3.2 has been removed from the rails-dev CI matrix.

• Drop support for Ruby 3.2 rails/rails#56511