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

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1. Gems need to be installed before we can run the
  gitlab:workhorse:install task
1. Update gitlab-shell after gitlab-workhorse since it's a manual step
  that needs to be done in `/home/git/gitlab-shell`
Signed-off-by: NRémy Coutable <remy@rymai.me>

This fixes a compatibility issue with Git 2.11:

- Issue: gitlab-org/gitlab-ce#25301
- gitlab-shell MR: gitlab-org/gitlab-shell!112
- CE MR: gitlab-org/gitlab-ce!7967
- EE MR: gitlab-org/gitlab-ee!964
Signed-off-by: NRémy Coutable <remy@rymai.me>

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This reverts merge request !7909

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Signed-off-by: NRémy Coutable <remy@rymai.me>

The task will use the canonical repo and the required version.
Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

v1.0.0 was mistakenly tagged with a lightweight, rather than annotated, tag,
which caused compiled versions of workhorse to wrongly report their version.

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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.

Fixes Go 1.5 compatibility.