Rename gem from omnibus-ctl to gitlab-omnibus-ctl

This will enable us to publish our own gem instead of installing
from Git source.

We'd still have to use `require: 'omnibus-ctl'`. This
will enable us to use `bundle` to isolate all Omnibus dependencies.

Relates to https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/8368

omnibus-ctl.gemspec

@@ -2,12 +2,12 @@ $:.push File.expand_path("lib", __dir__)
 require "omnibus-ctl/version"
 
 Gem::Specification.new do |s|
-  s.name        = "omnibus-ctl"
+  s.name        = "gitlab-omnibus-ctl"
   s.version     = Omnibus::Ctl::VERSION
   s.authors     = ["Chef Software, Inc."]
   s.email       = ["legal@chef.io"]
   s.licenses    = ["Apache-2.0"]
-  s.homepage    = "http://github.com/chef/omnibus-ctl"
+  s.homepage    = "https://gitlab.com/gitlab-org/build/omnibus-mirror/omnibus-ctl"
   s.summary     = %q{Provides command line control for omnibus packages}
   s.description = %q{Provides command line control for omnibus pakcages, rarely used as a gem}