DEV: Change hide_email_address_taken default to true (#30293)

We're changing the default of hide_email_address_taken to true. This is a trade-off we want to make, as it prevents account enumeration with minimal impact on legitimate users. If you forget you have an account and try to sign up again with the same e-mail you'll receive an e-mail letting you know.

spec/lib/email_updater_spec.rb

@@ -5,6 +5,8 @@ RSpec.describe EmailUpdater do
   let(:new_email) { "new.email@example.com" }
 
   it "provides better error message when a staged user has the same email" do
+    SiteSetting.hide_email_address_taken = false
+
     Fabricate(:user, staged: true, email: new_email)
 
     user = Fabricate(:user, email: old_email)