PERIOD should be a non-reserved keyword

[mikebin]

Describe the bug
When upgrading from ksqlDB 5.5.x to 6.0.x+, if an existing query uses a field named PERIOD, ksqlDB fails to parse the query in the command topic in the new version. Upgrades should be able to execute existing queries without any changes. PERIOD was added as a keyword in 6.0.x to support GRACE PERIOD, but was not added a non-reserved word, which leads to this issue.

To Reproduce
ksqlDB version: 6.0.x+

Define the following query in ksqlDB 5.5.x:

create stream s as select level as PERIOD from ksql_processing_log emit changes;

Upgade ksqlDB in place (keeping same command topic) to 6.0.x, and start the server.

Expected behavior
Server starts and executes all existing queries successfully.

Actual behaviour
Exception parsing statement in ksqlDB command topic:

[2022-01-12 13:32:22,465] INFO Incompatible command record detected when processing command topic (io.confluent.ksql.rest.server.computation.CommandRunner:382)
org.apache.kafka.common.errors.SerializationException: com.fasterxml.jackson.databind.JsonMappingException: line 1:10: mismatched input 'PERIOD' expecting {'EMIT', 'CHANGES', 'FINAL', 'ESCAPE', 'INTEGER', 'DATE', 'TIME', 'TIMESTAMP', 'INTERVAL', 'YEAR', 'MONTH', 'DAY', 'HOUR', 'MINUTE', 'SECOND', 'ZONE', 'PARTITION', 'STRUCT', 'EXPLAIN', 'ANALYZE', 'TYPE', 'TYPES', 'SHOW', 'TABLES', 'COLUMNS', 'COLUMN', 'PARTITIONS', 'FUNCTIONS', 'FUNCTION', 'ARRAY', 'MAP', 'SET', 'RESET', 'SESSION', 'KEY', 'SINK', 'SOURCE', 'PRIMARY', 'REPLACE', 'ASSERT', 'ADD', 'ALTER', 'IF', IDENTIFIER, DIGIT_IDENTIFIER, QUOTED_IDENTIFIER, BACKQUOTED_IDENTIFIER, VARIABLE} (through reference chain: io.confluent.ksql.rest.server.computation.Command["plan"]->io.confluent.ksql.engine.KsqlPlanV1["queryPlan"]->io.confluent.ksql.engine.QueryPlan["physicalPlan"]->io.confluent.ksql.execution.plan.StreamSink["source"]->io.confluent.ksql.execution.plan.StreamSelect["selectExpressions"]->java.util.ArrayList[0])
Caused by: com.fasterxml.jackson.databind.JsonMappingException: line 1:10: mismatched input 'PERIOD' expecting {'EMIT', 'CHANGES', 'FINAL', 'ESCAPE', 'INTEGER', 'DATE', 'TIME', 'TIMESTAMP', 'INTERVAL', 'YEAR', 'MONTH', 'DAY', 'HOUR', 'MINUTE', 'SECOND', 'ZONE', 'PARTITION', 'STRUCT', 'EXPLAIN', 'ANALYZE', 'TYPE', 'TYPES', 'SHOW', 'TABLES', 'COLUMNS', 'COLUMN', 'PARTITIONS', 'FUNCTIONS', 'FUNCTION', 'ARRAY', 'MAP', 'SET', 'RESET', 'SESSION', 'KEY', 'SINK', 'SOURCE', 'PRIMARY', 'REPLACE', 'ASSERT', 'ADD', 'ALTER', 'IF', IDENTIFIER, DIGIT_IDENTIFIER, QUOTED_IDENTIFIER, BACKQUOTED_IDENTIFIER, VARIABLE} (through reference chain: io.confluent.ksql.rest.server.computation.Command["plan"]->io.confluent.ksql.engine.KsqlPlanV1["queryPlan"]->io.confluent.ksql.engine.QueryPlan["physicalPlan"]->io.confluent.ksql.execution.plan.StreamSink["source"]->io.confluent.ksql.execution.plan.StreamSelect["selectExpressions"]->java.util.ArrayList[0])

Additional context