CAMEL-22760: Add 4.21 upgrade guide entry for Spring Kafka properties bridge

[Croway]

Summary

• Add upgrade guide entry documenting the new automatic bridging of spring.kafka.* properties to Camel Kafka component configuration
• Documents the bridged properties, precedence rules, and the toggle to disable the bridge

Depends on: apache/camel-spring-boot#1709 being merged first.

Test plan

• Verify the upgrade guide renders correctly

Claude Code on behalf of Croway

[github-actions]

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🧪 CI tested the following changed modules:

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