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

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"
 	}

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"
 			]

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")
 }