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>

docs/build.gradle

@@ -39,14 +39,10 @@ private List<SourceDirectorySet> findSourcesWithName(String... approximateNames)
             }.collect { it.kotlin }
 }
 
-Object callback = {
-    switch (true) {
-        case project.hasProperty("DOKKA_PATH"):
-            outputDirectory = project.property("DOKKA_PATH").toString()
-            break
-        case System.getenv("DOKKA_PATH") != null:
-            outputDirectory = System.getenv("DOKKA_PATH")
-            break
+dokka {
+    // aggregating every module's sources into one publication needs more heap than the worker default
+    dokkaGeneratorIsolation = ProcessIsolation {
+        it.maxHeapSize.set("4g")
     }
 
     dokkaSourceSets {
@@ -55,7 +51,7 @@ Object callback = {
 
             sourceLink {
                 localDirectory.set(file("../"))
-                remoteUrl.set(new URL("https://github.com/InsanusMokrassar/ktgbotapi"))
+                remoteUrl.set(new URI("https://github.com/InsanusMokrassar/ktgbotapi"))
                 remoteLineSuffix.set("#L")
             }
         }
@@ -73,6 +69,3 @@ Object callback = {
         }
     }
 }
-
-tasks.dokkaGfm(callback)
-tasks.dokkaHtml(callback)