Restore IntegrationFlowDefinition from previous

It turns out that some downstream projects are going to
support both Spring Boot 2.2 and 2.3 versions (Spring Cloud).
This way they are going to compile against Spring Integration 5.2,
but possible have a runtime against Spring Integration 5.3.

* Restore `IntegrationFlowDefinition` from the previous version
to keep binary compatibility at runtime

Author: artembilan