Add activestorage as dependency

We use ActiveStorage::Current in our BaseControllers
since a3119d5 but
we never depended on it.

For stores never having activestorage as dependency
this was breaking.

(cherry picked from commit d495087)

core/solidus_core.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
 
   %w[
     actionmailer actionpack actionview activejob activemodel activerecord
-    activesupport railties
+    activestorage activesupport railties
   ].each do |rails_dep|
     s.add_dependency rails_dep, ['>= 6.0', '< 7.1.x']
   end