Keep the credential_store vocabulary lists private

They sit after `private`, which does not apply to constants, so both are
public API today. Nothing outside the file reads them, and the setting
has not shipped, so this is the last release where making them private
costs nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hsbt

lib/rubygems/config_file.rb

@@ -853,7 +853,10 @@ def warn_credential_store_fallback
   end
 
   CREDENTIAL_STORE_OFF = %w[false 0 no off f n].freeze
+  private_constant :CREDENTIAL_STORE_OFF
+
   CREDENTIAL_STORE_ON = %w[true 1 yes on t y].freeze
+  private_constant :CREDENTIAL_STORE_ON
 
   #--
   # Anything that reads as a boolean is one, so RUBYGEMS_CREDENTIAL_STORE=0