Replaced the usage of toBeTruthy

What does this MR do and why?

Replaced the usage of toBeTruthy in spec/frontend/vue_shared/components/confirm_modal_spec.js

Resolves the issue

Screenshots or screen recordings

How to set up and validate locally

1. Navigate inside gitlab directory
2. Run yarn jest spec/frontend/vue_shared/components/confirm_modal_spec.js

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.

Related to #369795 (closed)