Merge branch 'docs-redis-ha-upgrade' into 'master'

Add docs about upgrading Redis HA setup

See merge request gitlab-org/omnibus-gitlab!3815
8 jobs for master in 57 minutes and 1 second
Status Job ID Name Coverage
  Prepare Assets
passed #400087168
docker
fetch-assets

00:03:58

 
  Trigger Package
passed #400087169
triggered-packages
Trigger:package

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  Trigger Docker
passed #400087170
docker
Trigger:gitlab-docker

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passed #400087171
docker
Trigger:qa-docker

00:12:17

 
  Trigger Ha Validate
manual #400087172
allowed to fail manual
Trigger:ha-validate
 
  Trigger Qa
passed #400087173
Trigger:qa-test

00:21:10

 
  Metrics
manual #400087174
docker allowed to fail manual
Trigger:measure-memory
 
  Notification Fail
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notify:slack-fail:scheduled-master