docs: Add missing enableRetry configuration to producerBuilderCustomizer DeadLetterPolicy example

[sundar-ka]

✅ Contribution Checklist

• I read the contribution guide
• I updated the versioned docs

[lhotari]

There's an .enableRetry(true) example in the "Retry letter topic" docs. Using the "dead letter topic" feature doesn't require enabling the "retry letter topic" feature so that's why the change could be misleading.
I'd suggest improving the descriptions of "retry letter topic" and "dead letter topic" to explain the connection between these 2 features.

[lhotari]

Sorry about the previous review. This change is fine. I updated it to target currently maintained document versions.