Changing gitlab-shell version to 3.6.6 instead of 3.6.3, later on the upgrade...

Changing gitlab-shell version to 3.6.6 instead of 3.6.3, later on the upgrade process it will complain about running 3.6.3 instead of 3.6.6.

doc/update/8.12-to-8.13.md

@@ -72,7 +72,7 @@ sudo -u git -H git checkout 8-13-stable-ee
 ```bash
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch --all --tags
-sudo -u git -H git checkout v3.6.3
+sudo -u git -H git checkout v3.6.6
 ```
 ### 6. Update gitlab-workhorse