Terminate the Chat Redis instance in favor of the shared Redis instance

Problem

In the investigation of 2025-01-23: All traffic dropped to redis-cluste... (#19161 - closed), it turned out that the Chat Redis instance is currently being used by Chat history feature, only. Since we're switching to PostgreSQL from Redis for persisting chat history, there is a chance that the dedicated Redis instance is no longer necessary.

Proposal

• Investigate the current usage of Gitlab::Redis::Chat.
  • If there is any, investigate if we can switch to the other Redis instance such as Gitlab::Redis::Cache.
• Introduce a feature flag to turn off the traffic to Gitlab::Redis::Chat. Rollout the change on gitlab.com and make sure that there are no incidents.
• Terminate the Chat Redis instances for gitlab.com.