[Gradle] Add reminder to remove kotlinExtension check in consumerApiUsage

The `kotlinExtension` check in consumerApiUsage function can be removed
when legacy MPP plugin `kotlin-platform-common` is also removed from
the code base.

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/mpp/KotlinUsages.kt

@@ -36,6 +36,7 @@ object KotlinUsages {
         when {
             platformType in jvmPlatformTypes -> JAVA_API
             platformType == common
+                    /** The kotlinExtension check below can be removed when legacy [KotlinPlatformCommonPlugin] is also removed. */
                     && project.kotlinExtension is KotlinMultiplatformExtension
                     && !project.isCompatibilityMetadataVariantEnabled -> KOTLIN_METADATA
             else -> KOTLIN_API