Geo: Add value transformation logic method

What does this MR do and why?

Simplify how gitlab's geo feature handles registry class name transformations, introduce a new method graphql_enum_key in the BaseRegistry class that encapsulates the value transformation logic.

This is a refactoring change and doesn't affect functionality.

Issue: #414370 (closed)

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ee/app/graphql/types/geo/registry_class_enum.rb

@@ -13,7 +13,8 @@ class RegistryClassEnum < BaseEnum
       ::Geo::Secondary::RegistryConsistencyWorker::REGISTRY_CLASSES.each do |registry_class|
         next unless registry_class.graphql_mutable?
 
-        value registry_class.to_s.gsub('Geo::', '').underscore.upcase,
+        # Disabling RuboCop: `graphql_enum_key` adheres to the uppercase rule of Graphql/EnumValues.
+        value registry_class.graphql_enum_key, # rubocop:disable Graphql/EnumValues
           value: registry_class.to_s,
           description: "#{registry_class} registry class"
       end