* Remove usage of removed `org.springframework.util.SocketUtils`
in favor of OS-selected port
* Migrate mail tests to Greenmail instead of already obsolete (and soon to be removed)
`org.springframework.integration.test.mail.TestMailServer`
* Use `MatcherAssert.assertThat()`
instead of deprecated `Assert.assertThat()`
* Use `IntegrationFlows.fromSupplier(Supplier)`
instead of removed `IntegrationFlows.from(Supplier)`
* Use `org.springframework.integration:spring-integration-test`
throughout the project for more testing utilities
