Memoize replica set check; listener fixes and cleanup

[washu]

This PR:

• Memoizes replica_set_configured? so the MongoDB topology check runs only once per process (reduces round-trips).
• Removes redundant logger / attr_reader :server overrides now provided by ActionCable::SubscriptionAdapter::Base.
• Fixes listener backoff behavior: avoid sleeping on expected Change Stream timeouts and guard enum usage to prevent NoMethodError rescues.
• Improves thread-safety by synchronizing access to @subscribers when building pipelines and handling documents.
• Makes validate_replica_set! raise ReplicaSetRequiredError when configured to require a replica set.
• Updates specs to reflect behavior and fixes a flaky change-stream test.
• Updates .rubocop.yml TargetRubyVersion and README typo.
  All tests pass (48 examples, 0 failures) and I ran the benchmark locally with results in the PR comment.