Prefer explicit association definition in requirements factory

What does this MR do and why?

This way, build(:requirements) or build_stubbed(:requirements) does not create any database records.

See https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#factories

Prefer implicit or explicit association definitions instead of using create / build for association setup. See issue #262624 for further context.

Refs !97183 (comment 1116348020)

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.