Update dependencies and migrate to Gradle 9 / Dokka v2

Dependency bumps: Kotlin 2.3.20->2.3.21, Coroutines 1.10.2->1.11.0,
Ktor 3.4.2->3.5.1, KSP 2.3.6->2.3.9, MicroUtils 0.29.2->0.30.0,
Dokka 2.0.0->2.2.0, Versions plugin 0.53.0->0.54.0, NMCP 1.4.4->1.6.0.

Gradle wrapper 8.13->9.6.1 with build-script fixes for Gradle 9:
Project.exec() -> providers.exec(), tasks.whenTaskAdded -> tasks.configureEach.

Dokka migrated to the Dokka Gradle Plugin v2. The docs module is now
included as the KDocs aggregator (dokka {} DSL with sourceRoots, new URI,
ProcessIsolation heap for the generator worker), excluded from binary
compatibility validation, and the kdocs workflow runs :docs:dokkaGenerate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

build.gradle

@@ -21,6 +21,11 @@ plugins {
     alias(libs.plugins.nmcp.aggregation)
 }
 
+// docs is a dokka-only aggregator module, exclude it from binary compatibility validation
+apiValidation {
+    ignoredProjects += ["docs"]
+}
+
 
 if ((project.hasProperty('SONATYPE_USER') || System.getenv('SONATYPE_USER') != null) && (project.hasProperty('SONATYPE_PASSWORD') || System.getenv('SONATYPE_PASSWORD') != null)) {
     nmcpAggregation {
@@ -44,7 +49,7 @@ allprojects {
         mavenLocal()
     }
     if (it != rootProject.findProject("docs")) {
-        tasks.whenTaskAdded { task ->
+        tasks.configureEach { task ->
             if(task.name == "jsLegacyBrowserTest" || task.name == "jsLegacyNodeTest") {
                 task.enabled = false
             }
@@ -54,13 +59,9 @@ allprojects {
 apply from: "./extensions.gradle"
 
 private String getCurrentVersionChangelog() {
-    OutputStream changelogDataOS = new ByteArrayOutputStream()
-    exec {
-        standardOutput = changelogDataOS
+    return providers.exec {
         commandLine './changelog_info_retriever', "$library_version", 'CHANGELOG.md'
-    }
-
-    return changelogDataOS.toString().trim()
+    }.standardOutput.asText.get().trim()
 }