Exp syntax: deprecate case-insensitivity of NULL and booleans

[andrus]

Let's deprecate the use of uppercase literals for NULL, TRUE, FALSE in the expression grammar. They should still work, but a warning should be given in the logs for their use, similar to how we warn about the use of legacy query params:

LOGGER.info("*** 'cayenneExp' control parameter is deprecated in protocol v1.1 (Agrest 4.1). Consider replacing it with 'exp'");