Optimize factory usage user entity spec

What does this MR do and why?

This MR replaces create with build_stubbed in spec/serializers/user_entity_spec.rb to optimize the factory usage as recommended in our handbook.

Average time of 5 runs before update: 12.48
Average time of 5 runs after the update: 11.972
🚀 Performance improved!
Speedup: 0.51s (4.1% faster)

This MR also adds the missing feature category in this spec.

References

Relates to: #378910.

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