Update input-kafka.asciidoc to indicate required version for Kafka 4.0

[strawgate]

Proposed commit message

Update input-kafka.asciidoc to indicate required version for Kafka 4.0

Related issues

#41473

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @strawgate? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[pierrehilbert]

Are we still targeting 9.0 for that?
If this is still the case, we should backport this to 8.x

[strawgate]

Are we still targeting 9.0 for that?
If this is still the case, we should backport this to 8.x

This should be backport to 8.x for sure as its just a docs change and will work with 8.x, I think the updates to the Samara library and changing the default value for this are both candidates for work to be included in 9.x