Fix different initial value warning in custom email verification

What does this MR do and why?

Running specs for ServiceDesk::CustomEmailVerification displays the following warning state_machine:

Both ServiceDesk::CustomEmailVerification and its :state machine have defined a different default for "state". Use only one or the other for defining defaults to avoid unexpected behaviors.

This MR tries to get rid of this warning message by overriding the default the state machine should use.

Follow-up of !118523 (comment 1381045619).

Screenshots or screen recordings

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How to set up and validate locally

1. Run bin/rspec spec/models/service_desk/custom_email_verification_spec.rb in gitlab folder

MR acceptance checklist

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