tests/multi-server: drop kafka-produce from the CI short suite

The kafka-produce multi-server test runs green locally against a
docker-compose redpanda, but on the shared GitHub Actions runners the
redpanda container fails to reach a healthy state reliably, even with
Redpanda's own dev-container preset.  Rather than leave a perennially
red CI job, rename short.ci.test.yml to short.test.yml so the test
runs from `make test.multi-server` but not from
`test.multi-server.ci`.  Local runs and future dedicated kafka
infrastructure can still exercise it.

Author: arr2036