https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#step-1-update-the-installation-from-source-guide

In step 1 and step 2

Install gitlab-shell and gitlab-workhorse after updating the code, and
update configuration files before installing libs, migrations etc.

[ci skip]
Signed-off-by: NRémy Coutable <remy@rymai.me>

[ci skip]

We were missing some info on updating /etc/default/gitlab

In particular, changes needed to be made in order for Pages to work,
see https://gitlab.com/gitlab-org/gitlab-ce/issues/29372

Missed removing one of the redundant lines

Fix redundant wording.
Clean up code blocks and links.

And bump the stable branch version in the installation from source guide

This will be necessary when adding gitaly settings. This version
doesn't make any functional changes, but allows us to include this
breaking change in 9.0 and add the needed extra settings in the future
with backwards compatibility