Fix deprecation warnings

* 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

gradle.properties

@@ -1,3 +1,3 @@
-version=5.3.0.RELEASE
-springBootVersion=2.3.1.RELEASE
+version=5.4.0
+springBootVersion=2.4.0-SNAPSHOT
 org.gradle.jvmargs=-Xmx1536m