I18nCache: Fix overtaken class var RuntimeError in Ruby 3.0

This fixes errors like the following in Ruby 3.0:

    RuntimeError: class variable @@translate_required_html of SimpleForm::Inputs::StringInput is overtaken by SimpleForm::Inputs::Base
      .../simple_form/lib/simple_form/i18n_cache.rb:13:in `class_variable_get'
      .../simple_form/lib/simple_form/i18n_cache.rb:13:in `get_i18n_cache'
      .../simple_form/lib/simple_form/i18n_cache.rb:8:in `i18n_cache'
      .../simple_form/lib/simple_form/components/labels.rb:9:in `translate_required_html'
      .../simple_form/lib/simple_form/components/labels.rb:71:in `required_label_text'
      .../simple_form/lib/simple_form/components/labels.rb:43:in `label_text'
      .../simple_form/lib/simple_form/components/labels.rb:35:in `label'

lib/simple_form/i18n_cache.rb

@@ -5,19 +5,29 @@ module SimpleForm
   # caching facility to speed up form construction.
   module I18nCache
     def i18n_cache(key)
-      get_i18n_cache(key)[I18n.locale] ||= yield.freeze
+      store = I18nCacheStore.instance
+      store[key] ||= {}
+      store[key][I18n.locale] ||= yield.freeze
     end
 
-    def get_i18n_cache(key)
-      if class_variable_defined?(:"@@#{key}")
-        class_variable_get(:"@@#{key}")
-      else
-        reset_i18n_cache(key)
-      end
+    def reset_i18n_cache(key)
+      I18nCacheStore.instance[key] = {}
     end
+  end
 
-    def reset_i18n_cache(key)
-      class_variable_set(:"@@#{key}", {})
+  class I18nCacheStore
+    include Singleton
+
+    def initialize
+      @store ||= {}
+    end
+
+    def [](key)
+      @store[key]
+    end
+
+    def []=(key, value)
+      @store[key] = value
     end
   end
 end