Gitaly signed certs option is deprecated

Team members for review and approval: Engineer(s): @pks-t | Tech Writer: @eread

This MR is a result of this discussion around the following issue:

• Rails' certificates may not be checked at all when <code data-sourcepos="7:56-7:72">self_signed_certs</code> is enabled

To be clear, the functionality is deprecated in Gitlab Shell, this issue is to clarify that the Gitaly setting is no longer applicable, and as such is created in an Extra block for the release post.