Main change is to bump .ruby-version to 2.6.3 so versions
are consistent for the GDK.

This brings in two improvements:

* Add support for using gl_project_path
* Provide expires_in for LFS if available

Pass push options along to gitlab-rails's post-receive endpoint

See https://gitlab.com/gitlab-org/gitlab-shell/blob/master/CHANGELOG

See https://gitlab.com/gitlab-org/gitlab-shell/blob/master/CHANGELOG

8.3.0 was tagged with the commit before the CHANGELOG.md and
version.rb bump, so this just fixes a harmless warning.

See https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/237 for more details.

Why and how to enable this is covered in the docs being changed
here. This requires gitlab-org/gitlab-shell@2e8b670 ("Add support for
SSH certificate authentication", 2018-06-14) which has been merged in
and tagged as 8.0.0, so GITLAB_SHELL_VERSION needs to be bumped.

Merging this closes gitlab-org/gitlab-ce#34572 see
gitlab-org/gitlab-shell!207 for the gitlab-shell MR.

Implementation notes:

 - The APIs being changed here are all internal, and their sole
   consumer is gitlab-shell.

 - Most of the changed code is a MR to gitlab-shell, see the
   gitlab-org/gitlab-shell!207 MR. That change covers why only some of
   the internal methods get a new "username" parameter, and why some
   others only get a "user_id".

Fixes a NoMethodError in the pre-receive hook: https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/206

This includes the change that prints the @username of a user instead
of the full name.

https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/204

gitlab-shell 5.10.3 does not introduce any new features; it just removes an
unused config option.