CAMEL-23132: Document ListObjectsV2 migration in upgrade guide

[Croway]

Summary

• Add camel-aws2-s3 section to the 4.21 upgrade guide documenting the ListObjects to ListObjectsV2 breaking change for pojoRequest=true users

I noticed this issue due to a failing test in camel spring boot repository apache/camel-spring-boot#1786

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🧪 CI tested the following changed modules:

• docs

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