Update Kotlin to 2.0 (#900)

- update Kotlin from 1.7.10 to 2.0.21
  - Kotlin 1.6 dependencies in Gradle lock files are expected because kotlinc,
    which is also used by some tests, internally uses some 1.6 dependencies
    for backwards compatibility reasons.
- update kotlinx-html and kotlinx-serialization
- adapt Kotlin code where necessary
- use Kotlin stdlib Path APIs where possible
- fix IntelliJ Kotlin inspection warnings
- reformat code with `./gradlew spotlessApply`
  - ktfmt adds lots of trailing commas
- Add workaround to fix IntelliJ "unresolved reference" errors

buildSrc/src/main/kotlin/pklAllProjects.gradle.kts

@@ -15,6 +15,7 @@
  */
 import com.diffplug.gradle.spotless.KotlinGradleExtension
 import org.gradle.accessors.dm.LibrariesForLibs
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 plugins { id("com.diffplug.spotless") }
@@ -48,9 +49,9 @@ plugins.withType(JavaPlugin::class).configureEach {
 }
 
 tasks.withType<KotlinCompile>().configureEach {
-  kotlinOptions {
-    jvmTarget = "17"
-    freeCompilerArgs = freeCompilerArgs + listOf("-Xjsr305=strict", "-Xjvm-default=all")
+  compilerOptions {
+    jvmTarget = JvmTarget.JVM_17
+    freeCompilerArgs.addAll("-Xjsr305=strict", "-Xjvm-default=all")
   }
 }