refactor: consume the config-file writer from stackable-operator

Replace the vendored Java-properties writer
(rust/operator-binary/src/config/writer.rs) with
stackable_operator::v2::config_file_writer (moved there via operator-rs
#1217 on the smooth-operator branch). Kafka's copy was the java-only subset
of the canonical hdfs writer; the upstream module's additional to_hadoop_xml
simply goes unimported. Drop the now-unused java-properties dependency.

No behaviour change; rendered .properties output is byte-identical by
construction (same code, new home).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: adwk67