Merge branch 'update_guide' into 'master'

Added update guide for 8.11

## What does this MR do?

I need to add notes about Elastic on EE side. To not create ugly conflicts in the future and to not duplicate work I created it here first.

I followed this guide https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#gitlab-ce

See merge request !5545