DEV: Fix deprecation warning after updating to messabe_bus 4.0.0.

DEV: Fix deprecation warning after updating to messabe_bus 4.0.0.

MessageBus.reliable_pub_sub is deprecated, use MessageBus.backend_instance instead.

Follow-up to aee9fcd2571516ba3c852a1858bef2e4f605cc37

diff --git a/lib/presence_channel.rb b/lib/presence_channel.rb
index f2e8031..f6f9e7e 100644
--- a/lib/presence_channel.rb
+++ b/lib/presence_channel.rb
@@ -251,7 +251,7 @@ class PresenceChannel
   # installations, this is the same Redis server as `Discourse.redis`.
   def self.redis
     if MessageBus.backend == :redis
-      MessageBus.reliable_pub_sub.send(:pub_redis) # TODO: avoid a private API?
+      MessageBus.backend_instance.send(:pub_redis) # TODO: avoid a private API?
     elsif Rails.env.test?
       Discourse.redis.without_namespace
     else
@@ -408,7 +408,7 @@ class PresenceChannel
 
     # TODO: Avoid using private MessageBus methods here
     encoded_channel_name = MessageBus.send(:encode_channel_name, message_bus_channel_name)
-    MessageBus.reliable_pub_sub.send(:backlog_id_key, encoded_channel_name)
+    MessageBus.backend_instance.send(:backlog_id_key, encoded_channel_name)
   end
 
   def redis_keys

GitHub sha: a407d4298441ab60dc8c175e0669b85b8628f8b4

This commit appears in #15562 which was approved by techAPJ. It was merged by tgxworld.