Add missing feature categories to deployment fixture specs

What does this MR do and why?

Adds missing feature_category: RSpec metadata to frontend fixture specs for deployment management.

Part of #584630 (closed)

Changes

File Feature Category
ee/spec/frontend/fixtures/deployments.rb deployment_management
spec/frontend/fixtures/clusters.rb deployment_management

Feature categories based on config/feature_categories.yml.

🛠️ with ❤️ at Siemens

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