Remove legacy --zookeeper option from example instructions

Several examples still show the legacy zookeeper options in the command for listing all topics. With the removal of the ZooKeeper dependency from Kafka (it's WIP as of Kafka 3.0), this option got first removed and doesn't exist any longer in the kafka-topics.sh script. Instead, the bootstrap-server=<KAFKA URL> option must be specified as the means for the tool for finding the Kafka cluster. While a nuisance before, showing the old option becomes a problem when updating the examples to Debezium 1.8, whose container image for Kafka is based on version 3.0.

Update: this also concerns the SCHEMA_REGISTRY_KAFKASTORE_CONNECTION_URL environment variable, which should be removed from several Compose files (see https://github.com/debezium/debezium-examples/pull/198/). So the following should be done altogether:

• Replace the --zookeeper option with --boostrap-server
• Remove SCHEMA_REGISTRY_KAFKASTORE_CONNECTION_URL
• {}Pin the version of the Confluent registry image
• Update the examples affected by the registry change to Debezium 1.8 (i.e. version in READMEs essentially) and verify that the given instructions still work; some of the examples are severely outdated also otherwise (e.g. the end-to-end one), but if we just could move them to the latest Debezium (and thus Kafka 3.0), that'd be great