Deprecate omnibus_gitconfig for removal in 17.0

Omnibus contains a configuration option omnibus_gitconfig which was historically used to pass git configuration options. Nowadays Gitaly has gitaly[:configuration][:git][:config] where the git configuration should go. There's currently fallback code in Omnibus to place the configuration from omnibus_gitconfig to the correct key. Since the legacy key is no longer needed, let's deprecate it and remove it in 17.0 so we can get rid of the fallback code.

From omnibus-gitlab!6866 (comment 1386452121)

Assigning the furthest milestone we have for now so it doesn't fall of the radar.