Fixes https://github.com/spring-projects/spring-integration/issues/3623
* `IntegrationFlow` refactoring
* Apply several code style improvements and good practices
* Code style: no empty lines for methods javadocs
* make deprecated implementation reuse actual one instead of the copy-paste approach
* add whats-new comments
* Fix whats-new page according to standards
* Add Java DSL for R2DBC module
* Add JavaDocs for R2DBC component setters
* Introduce a `R2dbcMessageSource.SelectCreator` to narrow
a `StatementMapper` API access from SpEL
* Add missed `package-info.java` into R2DBC packages
* Add `hamcrest-core` as test dependency into SI-R2DBC
since Awaitility doesn't work without it
* * Add `@NonNullApi` to R2DBC packages
The R2DBC channel adapters have wrong componentType
* Fix R2DBC component type to not mention `reactive-`: they are reactive by R2DBC definition
The `ReactiveRedisStreamMessageProducer` calls ack() explicitly
when populates `IntegrationMessageHeaderAccessor.ACKNOWLEDGMENT_CALLBACK` header,
but must provide an implementation of `SimpleAcknowledgment` instead
* Fix `ReactiveRedisStreamMessageProducer` to populate the proper
`IntegrationMessageHeaderAccessor.ACKNOWLEDGMENT_CALLBACK` header and in the proper moment
* Fix `ReactiveRedisStreamMessageProducerTests` to verify that `autoAck=false` works as expected
together with the `IntegrationMessageHeaderAccessor.ACKNOWLEDGMENT_CALLBACK` header functionality
* Implement `Emission` handling in the `FluxMessageChannel` and `IntegrationReactiveUtils`
* Upgrade to Spring Kafka `2.6.0`
* Fix R2DBC components for deprecation in the Spring Data R2DBC
* Implement `StatementMapper.SelectSpec` for query expression
* Clean up for some sporadic test failures
JIRA: https://jira.spring.io/browse/INT-4566
* Rework UPDATE logic according deprecations
* Use `ColumnMapRowMapper` for default `Map` payload type
* Clean up tests
* Remove redundant `@SuppressWarnings("deprecation")`
* Add `Duration.ofSeconds(10)` to `StepVerifier.verify()`
to avoid infinite wait and lose failing text context on the hang CI build
