Fix for #5116 does not handle all cases

[mikebin]

Describe the bug
The fix for #5116, which is included in ksqlDB 6.0.0, does not handle all cases. When accessing a pre-5.5.1 CP cluster (5.2.3), the following exception occurs:

[2020-09-30 16:39:50,693] ERROR Failed to start KSQL (io.confluent.ksql.rest.server.KsqlServerMain:66)
io.confluent.ksql.util.KsqlServerException: Could not get Kafka authorized operations!
at io.confluent.ksql.services.KafkaClusterUtil.isAuthorizedOperationsSupported(KafkaClusterUtil.java:57)
at io.confluent.ksql.security.KsqlAuthorizationValidatorFactory.create(KsqlAuthorizationValidatorFactory.java:52)
at io.confluent.ksql.rest.server.KsqlRestApplication.buildApplication(KsqlRestApplication.java:668)
at io.confluent.ksql.rest.server.KsqlRestApplication.buildApplication(KsqlRestApplication.java:583)
at io.confluent.ksql.rest.server.KsqlServerMain.createExecutable(KsqlServerMain.java:118)
at io.confluent.ksql.rest.server.KsqlServerMain.main(KsqlServerMain.java:59)
Caused by: java.util.concurrent.ExecutionException: org.apache.kafka.common.errors.UnsupportedVersionException: Attempted to write a non-default includeClusterAuthorizedOperations at version 7
at org.apache.kafka.common.internals.KafkaFutureImpl.wrapAndThrow(KafkaFutureImpl.java:45)
at org.apache.kafka.common.internals.KafkaFutureImpl.access$000(KafkaFutureImpl.java:32)
at org.apache.kafka.common.internals.KafkaFutureImpl$SingleWaiter.await(KafkaFutureImpl.java:89)
at org.apache.kafka.common.internals.KafkaFutureImpl.get(KafkaFutureImpl.java:260)
at io.confluent.ksql.services.KafkaClusterUtil.isAuthorizedOperationsSupported(KafkaClusterUtil.java:53)
... 5 more
Caused by: org.apache.kafka.common.errors.UnsupportedVersionException: Attempted to write a non-default includeClusterAuthorizedOperations at version 7

Note that UnsupportedVersionException is wrapped in an ExecutionException, and the code currently only checks for UnsupportedVersionException: https://github.com/confluentinc/ksql/blob/6.0.0-post/ksqldb-engine/src/main/java/io/confluent/ksql/services/KafkaClusterUtil.java#L48-L58

To Reproduce
Startup ksqlDB 6.0.0 against a pre-CP 5.5.1 cluster (for example, 5.2.3).

Expected behavior
Ignore the unsupported version exception

Actual behaviour
Exception not being properly caught and ignored, causing ksqlDB server to fail at startup

Additional context
If authorization is not needed, setting ksql.access.validator.enable=false is a workaround. Upgrading brokers to CP 5.5.1+ is another workaround.