Update turbo-rails gem to v2.0

The new version adds cool new features such as page morphing and
refreshes. Specifically, we're going to use turbo stream refreshes
in a later commit.

admin/solidus_admin.gemspec

@@ -33,6 +33,6 @@ Gem::Specification.new do |s|
   s.add_dependency 'solidus_backend'
   s.add_dependency 'solidus_core', '> 4.2'
   s.add_dependency 'stimulus-rails', '~> 1.2'
-  s.add_dependency 'turbo-rails', '~> 1.4'
+  s.add_dependency 'turbo-rails', '~> 2.0'
   s.add_dependency 'view_component', '~> 3.9'
 end