Rename "generate trial" test spec describe block

What does this MR do and why?

Rename create_trial_account to generate trial for test spec describe block

Because the name doesn't match the method name.

How to set up and validate locally

bin/rspec -fd ee/spec/lib/gitlab/subscription_portal/clients/rest_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.