SpringBoot Clarify if we need to expose an environment property for Flyway

See https://github.com/flyway/flyway/issues/3732 and https://github.com/spring-projects/spring-boot/commit/19df3934c61d94f414aa40f380954e51f83bac7d#

Comment From: philwebb

We discussed this today as a team and given this comment we've decided to ignore this new property until someone asks us to support it.