Update redis.md with an updated required version

With GL 12.1 and later we require Redis 3.2 or later for
compatibility because we rely on sending an optional parameter
to SPOP.

doc/administration/high_availability/redis.md

@@ -8,8 +8,10 @@ type: reference
 
 The following are the requirements for providing your own Redis instance:
 
-- Redis version 2.8 or higher. Version 3.2 or higher is recommend as this is
-  what ships with the GitLab Omnibus package.
+- GitLab 12.0 and later requires Redis version 3.2 or higher. Version 3.2 or higher is recommend as this is
+  what ships with the GitLab Omnibus package. Older Redis versions do not
+  support an optional count argument to SPOP which is now required for
+  [Merge Trains](../../ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md).
 - Standalone Redis or Redis high availability with Sentinel are supported. Redis
   Cluster is not supported.
 - Managed Redis from cloud providers such as AWS Elasticache will work. If these

doc/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md

@@ -30,6 +30,7 @@ Pipelines invalidated through failures are immediately canceled and requeued.
 
 Merge trains have the following requirements and limitations:
 
+- GitLab 12.0 and later requires [Redis](https://redis.io/) 3.2 or higher to run Merge Trains.
 - This feature requires that
   [pipelines for merged results](../index.md#pipelines-for-merged-results-premium) are
   **configured properly**.