Upgrade Dokka to 1.5.31

Now publishes the html docs in the javadoc jar files as recommended by
dokka maintainers.

See: https://github.com/Kotlin/dokka/issues/1753

build.gradle.kts

@@ -45,17 +45,13 @@ subprojects {
         plugins.withType<KotlinMultiplatformPluginWrapper> {
             apply(plugin = "org.jetbrains.dokka")
 
-            val dokka by tasks.existing(DokkaTask::class) {
-                outputFormat = "javadoc"
-                outputDirectory = "$buildDir/docs/javadoc"
-            }
+            val dokkaHtml by tasks.existing(DokkaTask::class)
 
             val javadocJar by tasks.registering(Jar::class) {
                 group = LifecycleBasePlugin.BUILD_GROUP
                 description = "Assembles a jar archive containing the Javadoc API documentation."
                 archiveClassifier.set("javadoc")
-                dependsOn(dokka)
-                from(dokka.get().outputDirectory)
+                from(dokkaHtml)
             }
 
             configure<KotlinMultiplatformExtension> {

buildSrc/src/main/kotlin/Versions.kt

@@ -1,5 +1,5 @@
 object Versions {
-    const val dokka = "0.10.1"
+    const val dokka = "1.5.31"
     const val kotlin = "1.5.31"
     const val kotlinBenchmark = "0.3.1"
     const val kotlinCoroutines = "1.5.0"