Merge branch 'patch-6' into 'master'

Changed gitlab-shell version to avoid warning when precompiling the assets.

## What does this MR do?

Changes installation instructions from source to install v3.6.1 of gitlab-shell to avoid a pre-compile warning on gitlab.

See merge request !6747

doc/update/8.11-to-8.12.md

@@ -72,7 +72,7 @@ sudo -u git -H git checkout 8-12-stable-ee
 ```bash
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch --all --tags
-sudo -u git -H git checkout v3.6.0
+sudo -u git -H git checkout v3.6.1
 ```
 
 ### 6. Update gitlab-workhorse