Fix explicitApiMode could be overwritten by freeCompilerArgs

Now tasks has a separate task input for 'explicitApiMode' which is
passed to Kotlin compiler via arg, rather than free compiler args.

^KT-57653 Fixed

libraries/tools/kotlin-gradle-plugin-api/src/common/kotlin/org/jetbrains/kotlin/gradle/dsl/kotlinTopLevelExtensionConfig.kt

@@ -34,7 +34,10 @@ interface KotlinTopLevelExtensionConfig {
 /**
  * Different modes that can be used to set the level of issue reporting for [KotlinTopLevelExtensionConfig.explicitApi] option.
  */
-enum class ExplicitApiMode(val cliOption: String) {
+enum class ExplicitApiMode(
+    @Deprecated("Should not be exposed in api", level = DeprecationLevel.ERROR)
+    val cliOption: String
+) {
     /** Report issues as errors. */
     Strict("strict"),
 
@@ -44,5 +47,7 @@ enum class ExplicitApiMode(val cliOption: String) {
     /** Disable issues reporting. */
     Disabled("disable");
 
+    @Deprecated("Should not be exposed in api", level = DeprecationLevel.ERROR)
+    @Suppress("DEPRECATION_ERROR")
     fun toCompilerArg() = "-Xexplicit-api=$cliOption"
 }