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

docs/src/test/kotlin/DocSnippetTests.kt

@@ -38,7 +38,7 @@ class DocSnippetTestsEngine : HierarchicalTestEngine<DocSnippetTestsEngine.Execu
   private val projectDir = rootProjectDir.resolve("docs")
   private val docsDir = projectDir.resolve("modules")
 
-  companion object {
+  private companion object {
     val headingRegex = Regex("""(?u)^\s*(=++)\s*(.+)""")
     val collapsibleBlockRegex = Regex("""(?u)^\s*\[%collapsible""")
     val codeBlockRegex = Regex("""(?u)^\s*\[source(?:%(tested|parsed)(%error)?)?(?:,\{?([a-zA-Z-_]+)}?)?""")