chore(ci): add ruby 4.0 to the matrix

…riment

Refactor Experiment Storage In order to reduce the amount of Redis Calls made during a single experiment

See #735

docs: add cache invalidation warning for Redis Caching

Replace manual bundle install with ruby/setup-ruby's built-in bundler-cache
feature for better compatibility with Ruby 4.0+ security features.

- Add `inherit_mode: { merge: [Exclude] }` to inherit default exclusion patterns
- Remove explicit `vendor/**/*` exclusion (now inherited from default config)
- Remove obsolete `gemfiles/**/*` exclusion (gemfiles directory was removed)

This ensures that `vendor/bundle` is properly excluded when using
`bundler-cache: true` in GitHub Actions workflow, preventing RuboCop
from inspecting gem files and loading their .rubocop.yml configurations.

Fixes the issue where RuboCop tried to require 'rubocop-rake' from
gems installed in vendor/bundle.

 ci: use bundler-cache in setup-ruby and fix RuboCop configuration