Cleanup JVM toolchain implementation

Remove support for <Gradle 6.7 and fix extension method to accept
'JavaToolchainSpec' instead of 'Any'.

^KT-47924 Fixed

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

@@ -74,8 +74,4 @@ interface KotlinJavaToolchain {
             javaLauncher: Provider<JavaLauncher>
         )
     }
-
-    companion object {
-        val TOOLCHAIN_SUPPORTED_VERSION: GradleVersion = GradleVersion.version("6.7")
-    }
 }