Update Kafka docs to use `step certificate p12`

### Description

[Hello mTLS for Kafka](https://smallstep.com/hello-mtls/doc/combined/kafka/kafka-cli) requires the use of OpenSSL to create PKCS#12 certificates. But now `step` has now support for this kind of certificates using `step certificate p12` command, see https://github.com/smallstep/cli/pull/372. 

We can now change kafka docs to use step instead of OpenSSL.