Restore external documentation links for Dokka

- Restore external documentation links in spring-integration-core's
  `dokkaSourceSets` using the new `externalDocumentationLinks.
  register()` API instead of deprecated `externalDocumentationLink`
- Fix indentation of spring-integration-core's `dokka` configuration
  block to be properly scoped within the project
- Remove redundant `noJdkLink.set(false)` setting (false is default)
- Remove unnecessary root-level `dokka` configuration that was
  setting module name and output directory without effect
- Remove unused `dokkaGeneratePublicationHtml` task dependency from
  `docsZip` as `dokkaGenerate` already handles the generation
- Simplify dependency chain by relying solely on
  `:spring-integration-core:dokkaGenerate`
- Use root project var to determine root
- Move the out of core module
  This makes the dokka build available to all modules if necessary

Author: cppwfs

build.gradle

@@ -188,6 +188,8 @@ configure(javaProjects) { subproject ->
     apply plugin: 'kotlin'
     apply plugin: 'kotlin-spring'
     apply plugin: 'io.spring.nullability'
+    apply plugin: 'org.jetbrains.dokka'
+
 
     apply from: "${rootDir}/gradle/publish-maven.gradle"
 
@@ -393,6 +395,27 @@ configure(javaProjects) { subproject ->
 
     check.dependsOn checkClasspathForConflicts, javadoc
 
+
+    dokka {
+        dokkaPublications.html {
+            outputDirectory.set(file(rootProject.layout.buildDirectory.file('kdoc')))
+        }
+        dokkaSourceSets.configureEach {
+            sourceRoots.setFrom(file('src/main/kotlin'))
+            classpath.from(sourceSets['main'].runtimeClasspath)
+            externalDocumentationLinks.register("spring-integration-api") {
+                url.set(uri("https://docs.spring.io/spring-integration/docs/$version/api/"))
+                packageListUrl.set(file('build/docs/javadoc/element-list').toURI())
+            }
+            externalDocumentationLinks.register("reactor-core") {
+                url.set(uri('https://projectreactor.io/docs/core/release/api/'))
+            }
+            externalDocumentationLinks.register("reactor-streams") {
+                url.set(uri('https://www.reactive-streams.org/reactive-streams-1.0.3-javadoc/'))
+            }
+        }
+    }
+
     publishing {
         publications {
             mavenJava(MavenPublication) {
@@ -468,7 +491,6 @@ project('spring-integration-cassandra') {
 project('spring-integration-core') {
     description = 'Spring Integration Core'
 
-    apply plugin: 'org.jetbrains.dokka'
     apply plugin: 'com.google.protobuf'
 
     dependencies {
@@ -511,17 +533,6 @@ project('spring-integration-core') {
         }
     }
 
-dokka {
-        dokkaPublications.html {
-            outputDirectory.set(file('../build/kdoc'))
-        }
-        dokkaSourceSets.configureEach {
-            noJdkLink.set(false)
-            sourceRoots.setFrom(file('src/main/kotlin'))
-            classpath.from(sourceSets['main'].runtimeClasspath)
-        }
-    }
-
     protobuf {
         protoc {
             artifact = "com.google.protobuf:protoc:$protobufVersion"
@@ -1088,17 +1099,6 @@ tasks.register('api') {
     dependsOn javadoc
 }
 
-dokka {
-    moduleName.set('spring-integration')
-    dokkaPublications.html {
-        outputDirectory.set(file('build/kdoc'))
-    }
-}
-
-tasks.named('dokkaGeneratePublicationHtml') {
-    dependsOn 'api'
-}
-
 apply from: "${rootDir}/gradle/docs.gradle"
 
 tasks.register('schemaZip', Zip) {
@@ -1137,8 +1137,7 @@ tasks.register('schemaZip', Zip) {
 }
 
 tasks.register('docsZip', Zip) {
-    dependsOn 'dokkaGeneratePublicationHtml'
-    dependsOn ':spring-integration-core:dokkaGenerate'
+    dependsOn 'dokkaGenerate'
     group = 'Distribution'
     archiveClassifier = 'docs'
     description = "Builds -${archiveClassifier} archive containing api and reference " +