* GH-3946: Revise Router channelKeyFallback option
Fixes https://github.com/spring-projects/spring-integration/issues/3946
The `AbstractMappingMessageRouter` has both `resolutionRequired` and `channelKeyFallback`
as `true` by default.
End-users expects them to back off when they set a `defaultOutputChannel`.
They really want something similar to Java `switch` statement
* Change the logic in the `AbstractMappingMessageRouter` to reset `channelKeyFallback`
to `false` when `defaultOutputChannel` to avoid attempts to resolve channel from name,
but rather fallback to `defaultOutputChannel` as it states from th mentioned
Java `switch` statement experience
* Deprecate `RouterSpec.noChannelKeyFallback()` in favor of newly introduced `channelKeyFallback(boolean)`
* Call `channelKeyFallback(false)` from an overloaded `defaultOutputToParentFlow()`
to reflect the mentioned expected behavior in Java DSL as well.
* Respectively, deprecate `KotlinRouterSpec.noChannelKeyFallback()` wrapper
in favor of newly introduced `channelKeyFallback(channelKeyFallback: Boolean)`
* Remove redundant already `noChannelKeyFallback()` option in the `NoFallbackAllowedTests`
* Document the change and new behavior
* Fix `IntegrationGraphServerTests` to `setChannelKeyFallback(true)` explicitly
* Remove not relevant `default-output-channel` from the `DynamicRouterTests-context.xml`
* Reject an `AbstractMappingMessageRouter` configuration where `defaultOutputChannel` is provided
and both `channelKeyFallback` & `resolutionRequired` are set to `true`.
Such a state makes `defaultOutputChannel` as not reachable and may cause some confusions in target
applications.
* Remove `&` symbol from JavaDocs
* Fix `boolean` expression for `AbstractMappingMessageRouter` configuration check
* Fix `IntegrationGraphServerTests` for new router behavior
* Improve language in docs
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