Merge branch 'update/installation-from-source-guide-8-15' into 'master'

Update "Installation from source" guide for 8.15.0

Following the steps from https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1.

See merge request !8015

doc/install/installation.md

@@ -271,9 +271,9 @@ sudo usermod -aG redis git
 ### Clone the Source
 
     # Clone GitLab repository
-    sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-ce.git -b 8-14-stable gitlab
+    sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-ce.git -b 8-15-stable gitlab
 
-**Note:** You can change `8-14-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
+**Note:** You can change `8-15-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
 
 ### Configure It
 

doc/update/8.14-to-8.15.md

@@ -72,7 +72,7 @@ sudo -u git -H git checkout 8-15-stable-ee
 ```bash
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch --all --tags
-sudo -u git -H git checkout v4.0.0
+sudo -u git -H git checkout v4.0.3
 ```
 
 ### 6. Update gitlab-workhorse