Fix deprecation warnings from the redis gem

[sharang-d]

Fixes these warnings introduced in Redis 4.6.0

Pipelining commands on a Redis instance is deprecated and will be removed in Redis 5.0.0.

redis.multi do
  redis.get("key")
end

should be replaced by

redis.multi do |pipeline|
  pipeline.get("key")
end

(called from /bundle_path/ruby/3.0.0/gems/rpush-7.0.0/lib/rpush/daemon/store/redis.rb:170:in `block in pending_notification_ids'}

Similar commit in sidekiq: sidekiq/sidekiq@c792342

[aried3r]

Looks good to me, thank you!