FIX: Ensure PresenceChannel does not raise error during readonly (#22899)

PresenceChannel configuration is cached using redis. That cache is used, and sometimes repopulated, during normal GET requests. When the primary redis server was readonly, that `redis.set` call would raise an error and cause the entire request to fail. Instead, we should ignore the failure and continue without populating the cache.

lib/presence_channel.rb

@@ -314,7 +314,10 @@ class PresenceChannel
         else
           raise InvalidConfig.new "Expected PresenceChannel::Config or nil. Got a #{result.class.name}"
         end
-      PresenceChannel.redis.set(redis_key_config, to_cache, ex: CONFIG_CACHE_SECONDS)
+
+      DiscourseRedis.ignore_readonly do
+        PresenceChannel.redis.set(redis_key_config, to_cache, ex: CONFIG_CACHE_SECONDS)
+      end
 
       raise PresenceChannel::NotFound if result.nil?
       result