Use a connection pool for Rack::Attack's redis cache

[zachmargolis]

connection pool all the things!!

This reuses the same connection pool config as #7283 but maybe there's a world where we want a separate number?

[zachmargolis]

Looks like Rails will keep a single connection for this, instead of create a new redis connection each time, so we can close this for now, and re-open if we ever switch to a multithreaded model:

https://github.com/rails/rails/blob/d5b0b09b5ab5c7fa65b052778b7152b406396ec3/activesupport/lib/active_support/cache/redis_cache_store.rb#L157-L165