Add gitlab-shell LFS pure SSH protocol setting

What does this merge request do and why?

This MR adds a new GDK setting called gitlab_shell.lfs.pure_ssh_protocol which is used in the support/templates/gitlab-shell/config.yml.erb which ultimately creates <GDK_ROOT>/gitlab-shell/config.yml with the following content:

lfs:
  # https://gitlab.com/groups/gitlab-org/-/epics/11872, disabled by default.
  pure_ssh_protocol: <true|false>

Related:

• git-lfs-transfer: Create configuration setting ... (gitlab-shell#741 - closed)
• New lfs.pure_ssh_protocol setting (gitlab-shell!936 - merged)

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.