Address some build warnings and cleanup (#1412)

(cherry picked from commit a1c60897916f418726107fec80ad79b2a4b8d500)

source-api/build.gradle.kts

@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.gradle.ExperimentalKotlinGradlePluginApi
+
 plugins {
     id("mihon.library")
     kotlin("multiplatform")
@@ -36,6 +38,11 @@ kotlin {
             }
         }
     }
+
+    @OptIn(ExperimentalKotlinGradlePluginApi::class)
+    compilerOptions {
+        freeCompilerArgs.add("-Xexpect-actual-classes")
+    }
 }
 
 android {
@@ -45,11 +52,3 @@ android {
         consumerProguardFile("consumer-proguard.pro")
     }
 }
-
-tasks {
-    withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
-        compilerOptions.freeCompilerArgs.addAll(
-            "-Xexpect-actual-classes",
-        )
-    }
-}