Use compilerOptions DSL in Kotlin Multiplatform opt-in example

`LanguageSettingsBuilder` interface will be deprecated in the future. Instead, it is better to use the existing `compilerOptions` DSL.

Author: alibagherifam

docs/topics/opt-in-requirements.md

@@ -222,9 +222,9 @@ If your Gradle module is a multiplatform module, use the `optIn` method:
 <tab title="Kotlin" group-key="kotlin">
 
 ```kotlin
-sourceSets {
-    all {
-        languageSettings.optIn("org.mylibrary.OptInAnnotation")
+kotlin {
+    compilerOptions {
+        optIn.add("org.mylibrary.OptInAnnotation")
     }
 }
 ```
@@ -233,11 +233,9 @@ sourceSets {
 <tab title="Groovy" group-key="groovy">
 
 ```groovy
-sourceSets {
-    all {
-        languageSettings {
-            optIn('org.mylibrary.OptInAnnotation')
-        }
+kotlin {
+    compilerOptions {
+        optIn.add('org.mylibrary.OptInAnnotation')
     }
 }
 ```