Deprecate support for RxJava 1.x

Closes spring-projectsgh-19628

Author: rstoyanchev

spring-core/src/main/java/org/springframework/core/ReactiveAdapterRegistry.java

@@ -48,6 +48,9 @@
  * {@link CompletableFuture}, Java 9+ {@code Flow.Publisher}, and Kotlin
  * Coroutines' {@code Deferred} and {@code Flow}.
  *
+ * <p><strong>Note:</strong> As of Spring Framework 5.3, support for RxJava 1.x
+ * is deprecated in favor of RxJava 2 and 3.
+ *
  * @author Rossen Stoyanchev
  * @author Sebastien Deleuze
  * @since 5.0
@@ -77,7 +80,7 @@ public ReactiveAdapterRegistry() {
 		}
 		this.reactorPresent = reactorRegistered;
 
-		// RxJava1
+		// RxJava1 (deprecated)
 		if (ClassUtils.isPresent("rx.Observable", classLoader) &&
 				ClassUtils.isPresent("rx.RxReactiveStreams", classLoader)) {
 			new RxJava1Registrar().registerAdapters(this);

src/docs/asciidoc/web-reactive.adoc

@@ -57,7 +57,14 @@ For annotated controllers, WebFlux transparently adapts to the reactive library
 the application. This is done with the help of the
 {api-spring-framework}/core/ReactiveAdapterRegistry.html[`ReactiveAdapterRegistry`], which
 provides pluggable support for reactive library and other asynchronous types. The registry
-has built-in support for RxJava and `CompletableFuture`, but you can register others, too.
+has built-in support for RxJava 2/3, RxJava 1 (via RxJava Reactive Streams bridge), and
+`CompletableFuture`, but you can register others, too.
+
+[NOTE]
+====
+As of Spring Framework 5.3, support for RxJava 1 is deprecated.
+====
+
 
 For functional APIs (such as <<webflux-fn>>, the `WebClient`, and others), the general rules
 for WebFlux APIs apply -- `Flux` and `Mono` as return values and a Reactive Streams