Use new rspec syntax for deactivate_members_over_limit_service_spec.rb

What does this MR do and why?

These turn into failures if run the specs with Ruby 3

We are running a pipeline with Ruby 3, and recently updated rspec-mocks. To get these specs to pass we should use the syntax in this MR .

Similar to !86157 (merged)

How to set up and validate locally

This only changes a spec, just need a green pipeline

MR acceptance checklist

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