MINOR: KIP-909 follow-ups (port validation, dead code, doc, test mock)

[frankvicky]

Small review-follow-ups on top of the merged KIP-909 work (#21080,
#22897):

• BootstrapConfiguration.enabled(...): wrap the host/port check
  with try/catch IllegalArgumentException so a malformed port (e.g.
  host:99999999999999999999 that overflows Integer.parseInt) surfaces
  as ConfigException("Invalid port in ..."), matching the pre-existing
  behaviour in ClientUtils.parseAndValidateAddresses. Without this a
  NumberFormatException would leak out of client construction.

• ClientUtils.parseAndValidateAddresses(AbstractConfig): removed.
  This overload had no callers after the KIP-909 changes. The remaining
  two overloads ((List<String>, String) and (List<String>, ClientDnsLookup)) are still used by BrokerApiVersionsCommand and
  ConnectionStressWorker, which construct NetworkClient directly with
  BootstrapConfiguration.DISABLED and pre-resolved addresses.

• CommonClientConfigs.BOOTSTRAP_RESOLVE_TIMEOUT_MS_DOC: reworded
  from "resolve for the bootstrap server address" to "resolve DNS for the
  bootstrap server address" for clarity.

• KafkaProducerTest: replaced new ConsumerGroupMetadata("test-group") with
  mock(ConsumerGroupMetadata.class) +
  when(...groupId()).thenReturn("test-group") — the constructor is
  @Deprecated(forRemoval).

Reviewers: Chia-Ping Tsai chia7712@gmail.com

[chia7712]

LGTM

[chia7712]

The parseAddresses method is only used by NetworkClient, but NetworkClient is already too large to add more code to ...