WebFlux.fn RouterFunctionDsl#invoke and CoRouterFunctionDsl#invoke
were wrongly exposed on public API and have never been designed to
be used by end users, but rather only invoked from router { } or
coRouter { } builders.
To fix that, avoiding this method being accessible from the DSL and
for the sake of consistency with WebMvc.fn RouterFunctionDsl,
Spring Framework 5.2 turns public fun invoke() method to
an internal fun build() one.
As a consequence RouterFunctionDsl and CoRouterFunctionDsl are not
open anymore, they are expected to be extended via Kotlin
extensions if needed.
Closes gh-22736
2eb631aaad