[Website]: Warn about Kafka 3.9 behavior change about timestamps

[lucabelluccini]

Type of issue

None

What documentation page is affected

https://www.elastic.co/docs/reference/beats/filebeat/kafka-output

What happened?

Kafka 3.6+ introduces stricter timestamp validation via two new broker/topic-level properties:

• log.message.timestamp.before.max.ms
• log.message.timestamp.after.max.ms

These limit the time difference between the message timestamp (from Filebeat) and Kafka broker receive time, and will cause message rejection if exceeded — but only when: log.message.timestamp.type=CreateTime
These checks are ignored if: log.message.timestamp.type=LogAppendTime

Additional info

Might be a good fit to be added to:

• Beats Kafka output
• Logstash Kafka output plugin
• Elastic AGent Kafka output