Upgrade to RxJava 3.x and remove older generations

As Spring Framework removed support for RxJava 1.x and 2.x, we should do
the same and only provide dependency management for RxJava 3.x.

Closes gh-28212

Author: bclozel

Diff for: spring-boot-project/spring-boot-actuator/build.gradle

@@ -29,7 +29,6 @@ dependencies {
 	}
 	optional("io.r2dbc:r2dbc-pool")
 	optional("io.r2dbc:r2dbc-spi")
-	optional("io.reactivex:rxjava-reactive-streams")
 	optional("io.undertow:undertow-servlet-jakarta") {
 		exclude group: "org.jboss.spec.javax.annotation", module: "jboss-annotations-api_1.3_spec"
 	}

Diff for: spring-boot-project/spring-boot-dependencies/build.gradle

@@ -1127,22 +1127,8 @@ bom {
 			]
 		}
 	}
-	library("RxJava", "1.3.8") {
-		group("io.reactivex") {
-			modules = [
-				"rxjava"
-			]
-		}
-	}
-	library("RxJava Adapter", "1.2.1") {
-		group("io.reactivex") {
-			modules = [
-				"rxjava-reactive-streams"
-			]
-		}
-	}
-	library("RxJava2", "2.2.21") {
-		group("io.reactivex.rxjava2") {
+	library("RxJava3", "3.1.3") {
+		group("io.reactivex.rxjava3") {
 			modules = [
 				"rxjava"
 			]

Diff for: spring-boot-project/spring-boot-starters/spring-boot-starter-data-couchbase-reactive/build.gradle

@@ -7,6 +7,5 @@ description = "Starter for using Couchbase document-oriented database and Spring
 dependencies {
 	api(project(":spring-boot-project:spring-boot-starters:spring-boot-starter"))
 	api("io.projectreactor:reactor-core")
-	api("io.reactivex:rxjava-reactive-streams")
 	api("org.springframework.data:spring-data-couchbase")
 }