chore: Update review instructions

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

This MR updates the review instructions to apply some verbiage tweaks:

• Gherkin comments are now only suggested for complex tests, with an explicit note that they can be omitted for less complex tests
• Use of files to write HTTP Responses for mux servers are now only suggested for complex tests, with an explicit note that they can be omitted for less complex responses
• require is explicitly added as a note to the testify/assert package, since we are ok with contributors using both (we mostly want to avoid t.fatal)

Let me know what you all think!