Document the minimum supported Redis version

As discussed in
https://gitlab.com/gitlab-org/gitlab/-/issues/238216#note_398065789

doc/administration/redis/replication_and_failover_external.md

@@ -17,9 +17,8 @@ Omnibus GitLab package.
 
 The following are the requirements for providing your own Redis instance:
 
-- Redis version 5.0 or higher is recommended, as this is what ships with
-  Omnibus GitLab packages starting with GitLab 12.7.
-- GitLab 13.0 and later requires Redis version 4.0 or higher.
+- Find the minimum Redis version that is required in the
+  [requirements page](../../install/requirements.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/install/installation.md

@@ -401,10 +401,11 @@ Starting from GitLab 12.1, only PostgreSQL is supported. Since GitLab 13.0, we r
 
 ## 7. Redis
 
-GitLab requires at least Redis 5.0.
+NOTE: **Note:**
+See the [requirements page](requirements.md#redis-versions) for the minimum
+Redis requirements.
 
-If you are using Debian 10 or Ubuntu 20.04 and up, you can install
-Redis 5.0 with:
+Install Redis with:
 
 ```shell
 sudo apt-get install redis-server

doc/install/requirements.md

@@ -74,9 +74,12 @@ a version older than `v10.13.0`, you need to update it to a newer version. You
 can find instructions to install from community maintained packages or compile
 from source at the [Node.js website](https://nodejs.org/en/download/).
 
-## Redis versions
+### Redis versions
 
-GitLab requires Redis 5.0+. Beginning in GitLab 13.0, lower versions are not supported.
+GitLab 13.0 and later requires Redis version 4.0 or higher.
+
+Redis version 5.0 or higher is recommended, as this is what ships with
+[Omnibus GitLab](https://docs.gitlab.com/omnibus/) packages starting with GitLab 12.7.
 
 ## Hardware requirements