Deprecate the built-in kafka output and shift to an xk6 extension

[na--]

This is a notice to the k6 community that we plan to gracefully spin out the current k6 Apache Kafka built-in output as an xk6 output extension.

This will be done in the following steps:

1. We'll make a semi-official xk6 output extension repo with the current kafka output code
2. We'll start using that new repo as a dependency in k6 for the built-in kafka output (WIP PR at Deprecate kafka output and move the code to an extension #1917) in k6 v0.32.0
3. The "built-in" kafka output will still continue to work for the next 2-3 k6 versions, but it will show a deprecation warning to stop using it and point users to the xk6 kafka extension instead.
4. We will make improvements only to the kafka output extension, we won't change the k6 built-in one.
5. After 2-3 k6 versions (e.g. in k6 v0.34.0), we will completely drop the "built-in" kafka output.