Optimize factory usage in clusters/deployment_entity_spec

What does this MR do and why?

This MR replaces create with build_stubbed in ee/spec/serializers/clusters/deployment_entity_spec.rb to optimize factory usage as recommended in our handbook. This spec is part of the list .rubocop_todo/rspec/factory_bot/avoid_create.yml. It was verified that database persistence is not needed using the factory doctor command FDOC=1 bin/rspec ee/spec/serializers/clusters/deployment_entity_spec.rb.

Factory Usage Optimization Results

Before optimization:

• Factory time: 2.948s (17.54% of total time)

After optimization:

• Factory time: 0.368s (2.68% of total time)

Improvement:

• Factory time reduced by: 2.580s
• Factory time percentage reduced by: 14.86%
• Overall factory usage improvement: 87.52%

Factory Doctor Output

[TEST PROF INFO] FactoryDoctor report

Total (potentially) bad examples: 3
Total wasted time: 00:02.883

Clusters::DeploymentEntity (./ee/spec/serializers/clusters/deployment_entity_spec.rb:5) (21 records created, 00:02.883)
  exposes deployable name (./ee/spec/serializers/clusters/deployment_entity_spec.rb:18) – 7 records created, 00:01.721
  exposes id (./ee/spec/serializers/clusters/deployment_entity_spec.rb:10) – 7 records created, 00:00.577
  exposes iid (./ee/spec/serializers/clusters/deployment_entity_spec.rb:14) – 7 records created, 00:00.584

This MR also adds the missing feature_category to this spec.

References

Relates to: #378910

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