Restore 'KotlinGradleSubplugin' as marker interface.

This avoids build failure in 'buildSrc' module with 'kotlin-dsl' applied
with 'ClassNotFound' exception.

^KT-51378 Fixed

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinGradleSubplugin.kt

@@ -116,3 +116,10 @@ interface KotlinCompilerPluginSupportPlugin : Plugin<Project> {
 open class SubpluginArtifact(val groupId: String, val artifactId: String, val version: String? = null)
 
 class JetBrainsSubpluginArtifact(artifactId: String) : SubpluginArtifact(groupId = "org.jetbrains.kotlin", artifactId = artifactId)
+
+/**
+ * Marker interface left here for backward compatibility with older plugin versions.
+ * Remove once minimal supported Gradle version will use Kotlin 1.7+.
+ */
+@Deprecated(level = DeprecationLevel.HIDDEN, message = "")
+interface KotlinGradleSubplugin